Method for measuring density, printing method, method of calculating correction value, method of manufacturing printing apparatus and method for obtaining correction value

ABSTRACT

A method for measuring density, includes: forming on a medium a pattern that consists of a plurality of dot rows formed respectively in a plurality of row regions lined up in a direction intersecting a movement direction in which a plurality of nozzles move, by forming each of the dot rows in the row region arranged in the movement direction by ejecting ink from the nozzles; reading the pattern by a scanner; measuring density of each of the row regions of the read pattern; calculating respective modification values corresponding to each of the row regions, based on at least a part of a measurement result of the density of the plurality of the row regions; and modifying respective measured values of the density of each of the row regions based on the respective modification values corresponding to each of the row regions.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional of application Ser. No.11/412,936, filed Apr. 28, 2006, which claims priority from JapanesePatent Applications No. 2005-133699 and No. 2005-133701 filed on Apr.28, 2005, which are herein incorporated by reference.

BACKGROUND

1. Technical Field

The invention relates to a method for measuring density, a printingmethod, a method of calculating a correction value, a method ofmanufacturing a printing apparatus, and a method for obtaining acorrection value.

2. Related Art

There is known a printing apparatus which prints a print image on amedium (such as paper, cloth, and OHP film) by repeating alternately thefollowing actions: a dot formation action in which dots are formed onthe medium by ejecting ink from a head moving in a movement directionand a carrying action in which the medium is carried. The print imageprinted with the printing apparatus is formed by lining up in a carryingdirection a myriad of pieces of image which consist of dot rows.

The dot row which each piece of image consists of is formed by making anink droplet ejected from a nozzle of the head land on the medium. If anink droplet of ideal size lands on an ideal position, each of the dotrows is formed in their respective predetermined region (row region),and a piece of image with ideal density is formed in the region.However, actually, because of influence by variation in precision ofmanufacturing and the like, variation in density occurs among the piecesof image formed in the respective regions. As a result thereof, astreaky unevenness in density occurs in the print image.

Therefore, technologies for suppressing this unevenness in density andimproving print image quality are proposed (see JP-A-2-54676 andJP-A-6-166247, for example).

An image processing unit disclosed in JP-A-2-54676 performs sampling ofan image by a CCD sensor and outputs the digitized data through aninkjet printer. In order to correct unevenness in density, an imageprocessing unit disclosed in JP-A-2-54676 stores as coefficientscharacteristics of variation in gain of the CCD sensor andcharacteristics of unevenness in density of a head, and performsbinarization in contemplation of these coefficients.

In a method of correcting unevenness in recorded density which isdisclosed in JP-A-6-166247, patterns for detecting unevenness in densityare printed and unevenness in density is corrected based on density dataof the patterns for detecting unevenness in density.

JP-A-2-54676 does not disclose how to obtain coefficients reflectingcharacteristics of variation in gain of the CCD sensor. Accordingly,depending on a method for obtaining these coefficients, there are casesin which these coefficients cannot reflect characteristics of the CCDsensor properly. If these coefficients do not reflect characteristics ofthe CCD sensor properly, unevenness in density occurs in a print image.In JP-A-6-166247, after the patterns for detecting unevenness in densityare printed, the patterns for detecting unevenness in density are readby an image sensor, and density data is created. However, if an imagesensor cannot read the patterns for detecting unevenness in densityproperly, unevenness in density cannot be corrected properly andunevenness in density occurs in a print image.

SUMMARY

Therefore, an advantage of a method for measuring density of theinvention is to modify measured values of density properly. In addition,an advantage of a method for obtaining a correction value of theinvention is to obtain a correction value which is appropriate tocorrect unevenness in density. It should be noted that in a technologyfor correcting unevenness in density disclosed in JP-A-6-166247, imagedata is corrected based on a correction value corresponding to eachnozzle.

However, there are cases in which there is difference in density ofcolor even among pieces of image formed by the same nozzle. For example,there are cases in which there is difference in density of color evenamong pieces of image consisting of dot rows formed by the same nozzleif dot rows contiguous to each of the above-mentioned dot rows havedifferent characteristics. In this case, the correction valuecorresponding only to each nozzle cannot suppress unevenness in density.

Accordingly, in the invention, unevenness in density is suppressed bystoring a correction value corresponding to a row region in which a dotrow is to be formed and correcting density of each piece of imagedepending on the correction value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing the configuration of a printingsystem 100.

FIG. 2 is a block diagram showing the overall configuration of a printer1.

FIG. 3A is a schematic diagram showing the overall structure of theprinter 1. FIG. 3B is a horizontal sectional view of the overallstructure of the printer 1.

FIG. 4 is an explanatory diagram showing the arrangement of nozzles inthe lower surface of a head 41.

FIG. 5A is a vertical sectional view of a scanner 150. FIG. 5B is a planview of the scanner 150 with a lid 151 detached.

FIG. 6 is a flowchart of the processing during printing.

FIGS. 7A and 7B are explanatory diagrams of regular printing. FIG. 7Ashows positions of the head and how dots are formed in each of the passn through pass n+3, and FIG. 7B shows positions of the head and how dotsare formed in each of the pass n through pass n+4.

FIG. 8 is an explanatory diagram of front-end printing and rear-endprinting.

FIG. 9A is an explanatory diagram showing a state in which dots areformed ideally. FIG. 9B is an explanatory diagram showing how thevariation in precision of manufacturing among nozzles affects dotformation. FIG. 9C is an explanatory diagram showing how dots are formedby the printing method of the present embodiment.

FIG. 10 is a flowchart showing a process for obtaining correctionvalues, which is performed on an inspection process after a printer hasbeen manufactured.

FIG. 11 is an explanatory diagram showing a test pattern.

FIG. 12 is an explanatory diagram showing a correction pattern.

FIG. 13 is an explanatory diagram showing a reading range of acorrection pattern of cyan.

FIG. 14A is an explanatory diagram showing image data on detection of aninclination. FIG. 14B is an explanatory diagram showing how the locationof a top ruled line is detected. FIG. 14C is an explanatory diagramshowing rotated image data.

FIG. 15A is an explanatory diagram showing image data on cropping. FIG.15B is an explanatory diagram showing a crop line with respect to a topruled line. FIG. 15C is an explanatory diagram showing a crop line withrespect to a bottom ruled line.

FIG. 16 is an explanatory diagram showing how to convert resolution.

FIG. 17A is an explanatory diagram showing image data when a left ruledline is detected. FIG. 17B is an explanatory diagram showing how thelocation of the left ruled line is detected. FIG. 17C is an explanatorydiagram showing a density-measuring range of a belt-like pattern in thefirst row region formed with 30% of color density (CD).

FIG. 18 is a table of values of measured densities of five belt-likepatterns of cyan.

FIG. 19 is a graph showing measured values of belt-like patterns of cyanformed with 30%, 40% and 50% CD respectively.

FIG. 20A is an explanatory diagram showing a target designated tonevalue Sbt of a row region i for a designated tone value Sb. FIG. 20B isan explanatory diagram showing a target designated tone value Sbt of arow region j for a designated tone value Sb.

FIG. 21 is an explanatory diagram showing a table of correction valuesof cyan.

FIG. 22 is a flowchart showing processes under instructions by a user.

FIG. 23 is a flowchart showing processes in print data generation.

FIG. 24 is an explanatory diagram showing how to correct a density ofthe nth row region of cyan.

FIG. 25A is a graph of measured values in case that a scanner is innormal operation. FIG. 25B is a graph of measured values in case that ascanner is in abnormal operation.

FIGS. 26A and 26B show measured values arranged in order, which are usedon calculation of correction values. FIG. 26A is a graph in case that ascanner is in normal operation, and FIG. 26B is a graph in case that ascanner is in abnormal operation.

FIG. 27 is an explanatory diagram showing density around a boundarybetween the front-end print region and the regular print region anddensity around a boundary between the regular print region and therear-end print region.

FIG. 28 is a graph of measured values (average values) in the regularprint region which correspond to one cycle.

FIG. 29 is an explanatory diagram showing density of a regular printregion after density correction in case that a scanner is in abnormaloperation.

FIG. 30A is a graph of measured values before modification. FIG. 30B isa graph of measured values after modification.

FIG. 31A is a graph of correction values for comparison. FIG. 31B is agraph of correction values when a gradient of measured values exists inthe present embodiment.

FIG. 32A is an explanatory diagram showing correction values beforemodification. FIG. 32B is an explanatory diagram showing correctionvalues after modification.

FIG. 33A is an explanatory diagram showing correction values beforemodification. FIG. 33B is an explanatory diagram showing correctionvalues after modification.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The specification and the drawings describe at least the followings:

A method for measuring density, including:

-   -   forming on a medium a pattern that consists of a plurality of        dot rows formed respectively in a plurality of row regions lined        up in a direction intersecting a movement direction in which a        plurality of nozzles move, by forming each of the dot rows in        the row region arranged in the movement direction by ejecting        ink from the nozzles;    -   reading the pattern by a scanner;    -   measuring density of each of the row regions of the read        pattern;    -   calculating respective modification values corresponding to each        of the row regions, based on at least a part of a measurement        result of the density of the row regions; and    -   modifying respective measured values of the density of each of        the row regions based on the respective modification values        corresponding to each of the row regions.

This method for measuring density enables to modify the measured valuesproperly.

In the method for measuring density, it is desirable that the respectivemodification values corresponding to each of the row regions arecalculated based on a measurement result obtained by excluding ameasurement result of the row region located at an end section of thepattern from the above-mentioned measurement result of the density ofthe row regions. As a result thereof, the measured values can bemodified properly.

What is desirable is a method for measuring density in which a linearfitting line and an average value are obtained from the at least a partthe measurement result and in which the respective modification valuescorresponding to each of the row regions are calculated depending ondifference between a value of the linear fitting line in each of the rowregions and the average value. This enables to modify a gradientthroughout the measured values. In addition, it is preferable tocalculate the linear fitting line based on the least-square method. Thisenables to grasp the tendency of the gradient of the measured values.

What is desirable is a method for measuring density in which, if thepattern has a first dot row formed by first printing and a second dotrow formed by second printing that is different from the above-mentionedfirst printing, the at least a part of the measurement result includes ameasured value of density of the row region in which the first dot rowis to be formed and a measured value of density of the row region inwhich the second dot row is to be formed. This enables to calculate alinear fitting line that reflects both of first printing and secondprinting.

A printing method, including:

-   -   forming on a medium a pattern which consists of a plurality of        dot rows formed respectively in a plurality of row regions lined        up in a direction intersecting a movement direction in which a        plurality of nozzles move, by forming each of the dot rows in        the row region arranged in the movement direction by ejecting        ink from the nozzles;    -   reading the pattern by a scanner;    -   measuring density of each of the row regions of the read        pattern;    -   calculating respective modification values corresponding to each        of the row regions, based on at least a part of a measurement        result of the density of the row regions;    -   modifying respective measured values of the density of each of        the row regions based on the respective modification values        corresponding to each of the row regions;    -   calculating correction values corresponding respectively to the        row regions based on the respective modified measured values;        and    -   when forming a print image on a medium, forming dot rows that        the print image consists of, based on the correction values        corresponding respectively to the row regions in which the dot        rows are to be formed.

This printing method enables to form a print image without unevenness indensity.

What is desirable is a printing method in which the correction valuescorresponding respectively to a predetermined number of the row regionsare calculated, respectively and in which, when forming the print imageon the medium, the dot rows are formed by using the correction valuescorresponding respectively to the predetermined number of the row regionrepeatedly for each set of the predetermined number of the row regionsthat the print image consists of. Even in this case, the occurrence ofstreaks in a print image can be suppressed.

What is desirable is a printing method in which, when forming the printimage on the medium, a dot formation process in which the dot rows areformed and a carrying process in which the medium is carried with apredetermined carry amount are repeated and in which the correctionvalue corresponding to a certain row region is calculated based on themeasured value of density of the certain row region and the measuredvalue of density of another row region that is located an integermultiple of the carry amount from the above-mentioned certain rowregion. Even in this case, the occurrence of the difference in densityin a print image can be suppressed.

What is desirable is a printing method in which, when forming the printimage on the medium, the correction value corresponding to the certainrow region is used for forming a dot row to be formed in the certain rowregion and for forming a dot row to be formed in another row region thatis located an integer multiple of the carry amount from theabove-mentioned certain row region. As a result thereof, the number ofcorrection values to be stored can be reduced. This is effectiveespecially when the certain row region is located in the regular printregion.

What is desirable is a printing method in which the regular print regionof the pattern is smaller than the regular print region of the printimage. As a result thereof, the length of the pattern can be made short.

A method of calculating a correction value, including:

-   -   forming on a medium a pattern that consists of a plurality of        dot rows formed respectively in a plurality of row regions lined        up in a direction intersecting a movement direction in which a        plurality of nozzles move, by forming each of the dot rows in        the row region arranged in the movement direction by ejecting        ink from the nozzles;    -   reading the pattern by a scanner;    -   measuring density of each of the row regions of the read        pattern;    -   calculating respective modification values corresponding to each        of the row regions, based on at least a part of a measurement        result of the density of the row regions;    -   modifying respective measured values of the density of each of        the row regions based on the respective modification values        corresponding to each of the row regions; and    -   calculating correction values corresponding respectively to the        row regions based on the respective modified measured values.

This method of calculating a correction value enables to calculate aproper correction value.

A method of manufacturing a printing apparatus, including:

-   -   preparing a printing apparatus having a memory;    -   using the printing apparatus, forming on a medium a pattern that        consists of a plurality of dot rows formed respectively in a        plurality of row regions lined up in a direction intersecting a        movement direction in which a plurality of nozzles move, by        forming each of the dot rows in the row region arranged in the        movement direction by ejecting ink from the nozzles;    -   reading the pattern by a scanner;    -   measuring density of each of the row regions of the read        pattern;    -   calculating respective modification values corresponding to each        of the row regions, based on at least a part of a measurement        result of the density of the row regions;    -   modifying respective measured values of the density of each of        the row regions based on the respective modification values        corresponding to each of the row regions;    -   calculating correction values corresponding respectively to the        row regions based on the respective modified measured values;        and    -   storing the correction values in the memory

This method of manufacturing a printing apparatus enables to manufacturea printing apparatus which can suppress unevenness in density.

A method of manufacturing a printing apparatus, including:

-   -   preparing a printing apparatus having a memory;    -   using the printing apparatus, forming a pattern that consists of        a plurality of dot rows formed respectively in a plurality of        row regions lined up in a direction intersecting a movement        direction of nozzles;    -   reading the pattern by a scanner;    -   measuring density of each of the row regions of the read        pattern;    -   calculating a first correction value for correcting the density        of a row regions that is located in a first region of the        pattern, based on a measured value of the density of that row        region that is located in the first region;    -   calculating a second correction value for correcting the density        of a row region that is located in a second region contiguous to        the first region, based on a measured value of the density of        that row region and a measured value of the density of another        row region that is located in the second region;    -   modifying at least one of the first correction value and the        second correction value in order to reduce a difference between        the first correction value and the second correction value; and    -   storing the at least one modified correction value in the        memory.

If a correction value obtained by this method for obtaining a correctionvalue is used, print image quality can improve.

What is desirable is a method for obtaining a correction value in whichat least one of the first correction value and the second correctionvalue is modified in order to reduce a difference between an averagevalue of a plurality of the first correction values and an average valueof a plurality of the second correction values. In addition, it ispreferable that the difference between the average value of the firstcorrection values and the average value of the second correction valuesis determined as a modification value and that at least one of the firstcorrection value and the second correction value is modified based onthe modification value. As a result thereof, the difference in densityat the boundary between each of print regions can be reduced. However,in order to reduce a difference between the first correction value of arow region that is located in the first region and is contiguous to thesecond region and the second correction value of a row region that islocated in the second region and is contiguous to the first region, atleast one of the first correction value and the second correction valuecan be modified. In this case, it is preferable that the differencebetween the first correction value of the row region which is located inthe first region and is contiguous to the second region and the secondcorrection value of the row region which is located in the second regionand is contiguous to the first region is determined as a modificationvalue and that at least one of the first correction value and the secondcorrection value is modified based on the modification value.

What is desirable is a method for obtaining a correction value in whichthe measured values of the density of each of the row regions aremodified depending on the row regions and in which the first correctionvalue and the second correction value are calculated based on themodified measured values. In addition, it is preferable that, whenmodifying the measured values depending on the row regions, a linearfitting line and an average value are obtained from the measured values,and the measured values of the density of the row regions are modifieddepending on a difference between a value of the linear fitting line ineach of the row regions and the average value. Furthermore, it ispreferable to calculate the linear fitting line based on theleast-square method. As a result thereof, even if a gradient of themeasured values exists, a proper correction value can be obtained.

What is desirable is a method for obtaining a correction value in whichthe first region is a region including a first dot row formed by firstprinting and in which the second region is a region consisting of asecond dot row formed by second printing that is different from thefirst printing. As a result thereof, the difference in density at aboundary between different print regions that are formed by differentprinting methods can be reduced.

A printing method, including:

-   -   reading by a scanner a pattern that consists of a plurality of        dot rows formed respectively in a plurality of row regions lined        up in a direction intersecting a movement direction of nozzles;    -   measuring density of each of the row regions of the read        pattern;    -   calculating a first correction value for correcting the density        of a row region that is located in a first region of the        pattern, based on a measured value of the density of that row        region that is located in the first region;    -   calculating a second correction value for correcting the density        of a row region that is located in a second region contiguous to        the first region, based on a measured value of the density of        that row region and a measured value of the density of another        row region that is located in the second region;    -   modifying at least one of the first correction value and the        second correction value in order to reduce a difference between        the first correction value and the second correction value;    -   when forming a print image on a medium, forming a dot row that        is located in the first region and that the print image consists        of, based on the first correction value corresponding to the row        region in which that dot row is to be formed; and    -   forming a dot row that is located in the second region and that        the print image consists of, based on the second correction        value corresponding to the row region in which that dot row is        to be formed.

This printing method enables to improve print image quality.

A method of manufacturing a printing apparatus, including:

-   -   preparing a printing apparatus having a memory;    -   using the printing apparatus, forming a pattern that consists of        a plurality of dot rows formed respectively in a plurality of        row regions lined up in a direction intersecting a movement        direction of nozzles;    -   reading the pattern by a scanner;    -   measuring density of each of the row regions of the read        pattern;    -   calculating a first correction value for correcting the density        of a row regions that is located in a first region of the        pattern, based on a measured value of the density of that row        region that is located in the first region;    -   calculating a second correction value for correcting the density        of a row region that is located in a second region contiguous to        the first region, based on a measured value of the density of        that row region and a measured value of the density of another        row region that is located in the second region;    -   modifying at least one of the first correction value and the        second correction value in order to reduce a difference between        the first correction value and the second correction value; and    -   storing the at least one modified correction value in the        memory.

This method of manufacturing a printing apparatus enables to manufacturea printing apparatus having high image quality.

Configuration of Printing System

Printing System

FIG. 1 is an explanatory diagram showing the configuration of a printingsystem 100, which consists of at least a printing apparatus and aprinting control apparatus that controls operations of the printingapparatus. The printing system 100 of the present embodiment is providedwith a printer 1, a computer 110, a display device 120, input devices130, record/play devices 140, and a scanner 150.

The printer 1 is for printing images on a medium such as paper, cloth,film, and OHP film. The computer 110 is communicably connected to theprinter 1. In order to make the printer 1 print an image, the computer110 outputs print data corresponding to that image to the printer 1.This computer 110 has computer programs, such as an application programand a printer driver, installed thereon. A scanner driver is installedon the computer 110 and is for controlling the scanner 150 and forreceiving image data of a document read by the scanner 150.

Printer

FIG. 2 is a block diagram showing the overall configuration of theprinter 1. FIG. 3A is a schematic diagram showing the overall structureof the printer 1. FIG. 3B is a horizontal sectional view of the overallstructure of the printer 1. The basic structure of the printer accordingto the present embodiment is described below.

The printer 1 has a carry unit 20, a carriage unit 30, a head unit 40, adetector group 50, and a controller 60. The printer 1 receives printdata from the computer 110, which is an external device, and controlsthe various units (the carry unit 20, the carriage unit 30, and the headunit 40) through the controller 60. The controller 60 controls theseunits based on the print data received from the computer 110 to print animage on the paper. The detector group 50 monitors the conditions withinthe printer 1, and outputs the result of this detection to thecontroller 60. The controller 60 controls these units based on thisdetection result received from the detector group 50.

The carry unit 20 is for carrying a medium such as paper in apredetermined direction (hereinafter, referred to as the carryingdirection). The carry unit 20 has a paper supply roller 21, a carrymotor 22 (also referred to as “PF motor”), a carry roller 23, a platen24, and a paper discharge roller 25. The paper supply roller 21 is aroller for supplying, into the printer, paper that has been insertedinto a paper insert opening. The carry roller 23 is a roller forcarrying a paper S that has been supplied by the paper supply roller 21up to a printable region, and is driven by the carry motor 22. Theplaten 24 supports the paper S being printed. The paper discharge roller25 is a roller for discharging the paper S outside the printer, and isprovided on the downstream side in the carrying direction with respectto the printable region. The paper discharge roller 25 is rotated insynchronization with the carry roller 23.

The carriage unit 30 is for making a head move (also referred to as“scan”) in a predetermined direction (hereinafter, referred to as themovement direction). The carriage unit 30 has a carriage 31 and acarriage motor 32 (also referred to as “CR motor”). The carriage 31 canbe moved back and forth in the movement direction. The carriage 31detachably holds ink cartridges that contain ink. The carriage motor 32is a motor for moving the carriage 31 in the movement direction.

The head unit 40 is for ejecting ink onto the paper. The head unit 40has a head 41. The head 41 has a plurality of nozzles and intermittentlyejects ink from those nozzles. The head 41 is provided in the carriage31. Thus, when the carriage 31 moves in the movement direction, the head41 also moves in the movement direction. Dot rows (raster lines) areformed on the paper in the movement direction due to the head 41intermittently ejecting ink while moving in the movement direction.

FIG. 4 is an explanatory diagram showing the arrangement of the nozzlesin the lower surface of the head 41. A black ink nozzle group K, a cyanink nozzle group C, a magenta ink nozzle group M, and a yellow inknozzle group Y are formed in the lower surface of the head 41. Eachnozzle group is provided with a plurality of nozzles, which are ejectionopenings for ejecting ink of the respective colors. The plurality ofnozzles of each of the nozzle groups are arranged in rows at a constantspacing (nozzle pitch: k·D) in the carrying direction. Here, D is theminimum dot pitch in the carrying direction (that is, the spacingbetween dots formed on the paper S at maximum resolution). Further, k isan integer of 1 or more. For example, if the nozzle pitch is 180 dpi (1/180 inch), and the dot pitch in the carrying direction is 720 dpi (1/720 inch), then k=4. Each nozzle of each of the nozzle groups isassigned a number (#1 to #180) that becomes smaller as the nozzle isarranged more downstream. Each nozzle is provided with an ink chamber(not shown) and a piezo element. Driving the piezo element causes theink chamber to expand and contract, thereby ejecting an ink droplet fromthe nozzle.

The detector group 50 includes a linear encoder 51, a rotary encoder 52,a paper detection sensor 53, an optical sensor 54, and the like. Thelinear encoder 51 is for detecting the position of the carriage 31 inthe movement direction. The rotary encoder 52 is for detecting theamount of rotation of the carry roller 23. The paper detection sensor 53is for detecting the position of the front end of the paper to beprinted. The optical sensor 54 is attached to the carriage 31. Theoptical sensor 54 detects whether or not the paper is present, throughits light-receiving section detecting the reflected light of the lightthat has been irradiated onto the paper from its light-emitting section.

The controller 60 is a control section for carrying out control of theprinter. The controller 60 includes an interface section 61, a CPU 62, amemory 63, and a unit control circuit 64. The interface section 61 isfor exchanging data between the computer 110, which is an externaldevice, and the printer 1. The CPU 62 is a processing unit for carryingout overall control of the printer. The memory 63 is for ensuring aworking area and a storage area for the programs for the CPU 62, forinstance, and includes storage devices such as a RAM or an EEPROM. TheCPU 62 controls the various units via the unit control circuit 64 inaccordance with programs stored in the memory 63.

Scanner

FIG. 5A is a vertical sectional view of the scanner 150. FIG. 5B is aplan view of the scanner 150 with a lid 151 detached.

The scanner 150 is provided with the lid 151, a document platen glass152 on which a document 5 is placed, a reading carriage 153 that facesthe document 5 through the document platen glass 152 and that moves in asub-scanning direction, a guiding member 154 for guiding the readingcarriage 153 in the sub-scanning direction, a moving mechanism 155 formoving the reading carriage 153, and a scanner controller (not shown)that controls the various units of the scanner 150. The reading carriage153 has an exposure lamp 157 that shines light on the document 5, a linesensor 158 that detects a line image in a main scanning direction (inFIG. 5A, the direction normal to the surface of the paper on which thefigure is described), and optical devices 159 that lead the reflectedlight from the document 5 to the line sensor 158. Dashed lines in thereading carriage 153 shown in FIG. 5A show the path of light.

In order to read an image of the document 5, an operator raises the lid151, places the document 5 on the document platen glass 152, and lowersthe lid 151. The scanner controller moves the reading carriage 153 inthe sub-scanning direction with the exposure lamp 157 emitting light,and the line sensor 158 reads the image on a surface of the document 5.The scanner controller transmits the read image data to the scannerdriver installed on the computer 110, and thereby, the computer 110obtains the image data of the document 5.

Printing Method

Regarding Printing Operation

FIG. 6 is a flowchart of the processing during printing. The processesdescribed below are executed by the controller 60 controlling thevarious units in accordance with a program stored in the memory 63. Thisprogram includes codes for executing the various processes.

Receipt of Print Command (S001): The controller 60 receives a printcommand via the interface section 61 from the computer 110. This printcommand is included in a header of print data transmitted from thecomputer 110. The controller 60 then analyzes the content of the variouscommands included in the print data received, and performs the followingprocesses such as paper supply process, carrying process, and dotformation process by using the various units.

Paper Supply Process (S002): The paper supply process is a process forsupplying paper to be printed into the printer and positioning the paperat a print start position (also referred to as “indexed position”). Thecontroller 60 positions the paper at the print start position byrotating the paper supply roller 21 and the carry roller 23.

Dot Formation Process (S003): The dot formation process is a process forforming dots on the paper by ejecting ink intermittently from the head41 that moves in the movement direction. The controller 60 moves thecarriage 31 in the movement direction by driving the carriage motor 32,and then, while the carriage 31 is moving, causes the head 41 to ejectink in accordance with pixel data contained in the print data. Dots areformed on the paper when ink droplets ejected from the head 41 land onthe paper. Since ink is intermittently ejected from the head 41 that ismoving, dot rows (raster lines) consisting of a plurality of dots in themovement direction are formed on the paper.

Carrying Process (S004): The carrying process is a process for movingthe paper relative to the head in the carrying direction. The controller60 carries the paper in the carrying direction by rotating the carryroller 23. Due to this carrying process, the head 41 can form dots atpositions that are different from the positions of the dots formed inthe preceding dot formation process, in the next dot formation process.

Paper Discharge Determination (S005): The controller 60 determineswhether or not to discharge the paper being printed. The paper is notdischarged if there remains data to be printed on the paper beingprinted. The controller 60 gradually prints an image consisting of dotson the paper by repeating alternately the dot formation process andcarrying process until there is no more data to be printed.

Paper Discharge Process (S006): When there is no more data to be printedon the paper being printed, the controller 60 discharges the paper byrotating the paper discharge roller. It should be noted that whether ornot to discharge the paper can also be determined based on a paperdischarge command included in the print data.

Print Ending Determination (S007): Next, the controller 60 determineswhether or not to continue printing. If a next sheet of paper is to beprinted, then printing is continued and the paper supply process for thenext paper starts. If the next sheet of paper is not to be printed, thenthe printing operation is terminated.

Regarding Formation of Raster Lines

First, regular printing is described. The regular printing of thepresent embodiment is carried out using a printing method referred to asinterlaced printing. Here, “interlaced printing” means a printing schemein which, raster lines that are not recorded are sandwiched betweenraster lines that are recorded in one pass. A “pass” refers to one dotformation process, and “pass n” refers to the nth dot formation process.A “raster line” refers to a row of dots lined up in the movementdirection and is also referred to as “dot line”.

FIGS. 7A and 7B are explanatory diagrams of regular printing. FIG. 7Ashows positions of the head and how dots are formed in each of the passn through pass n+3, and FIG. 7B shows positions of the head and how dotsare formed in each of the pass n through pass n+4.

It should be noted that, for convenience's sake, only one of a pluralityof the nozzle groups is shown and the number of nozzles of each nozzlegroup is reduced. In addition, the head 41 (and the nozzle groups) isillustrated as if it is moving with respect to the paper, but thefigures merely show the relative positional relationship between thehead 41 and the paper, and in reality, the paper moves in the carryingdirection. Furthermore, for convenience of explanation, each nozzle isillustrated as if it forms only a few dots (circles in the figure), butin reality, there are numerous dots lined up in the movement direction(this row of dots is the raster line) because ink droplets areintermittently ejected from the nozzles that move in the movementdirection. As a matter of course, there are cases in which a dot is notformed depending on the pixel data. In the figure, a nozzle shown with afilled circle is a nozzle that is allowed to eject ink and a nozzleshown with a white circle is a nozzle that is not allowed to eject ink.Furthermore, in the figure, a dot shown with a filled circle is a dotthat is formed in the last pass and a dot shown with a white circle is adot that is formed in other passes therebefore.

In this interlaced printing, every time the paper is carried in thecarrying direction by a constant carry amount F, each nozzle records araster line immediately above another raster line that was recorded inthe immediately prior pass. In order to carry out recording with aconstant carry amount in this way, it is required (1) that the number N(integer) of nozzles that are allowed to eject ink is coprime to k and(2) that the carry amount F is set to N·D. Here, N=7, k=4, and F=7·D (D=1/720 inch).

However, there is a region in which raster lines can not be formedcontinuously in the carrying direction in case of using only thisregular printing. Therefore, printing methods which are respectivelyreferred to as front-end printing and rear-end printing are carried outrespectively before or after the regular printing.

FIG. 8 is an explanatory diagram of the front-end printing and rear-endprinting. The first five passes correspond to the front-end printing,and the last five passes correspond to the rear-end printing. In thefront-end printing, at the time when a part near the front end of theprint image is printed, the paper is carried by a smaller carry amount(1·D or 2·D) than the carry amount in the regular printing (7·D). Also,in the front-end printing, the nozzles that eject ink are not fixed. Inthe rear-end printing, in the same way as the front-end printing, at thetime when a part near the rear end of the print image is printed, thepaper is carried by a smaller carry amount (1·D or 2·D) than the carryamount in the regular printing (7·D). Also, in the rear-end printing, inthe same way as the front-end printing, the nozzles that eject ink arenot fixed. In this way, a plurality of raster lines lined upcontinuously in the carrying direction can be formed between the firstraster line and the last raster line.

A region in which raster lines are formed solely by the regular printingis referred to as a “regular print region”. A region which is located onthe front-end side of the paper (the downstream side in the carryingdirection) with respect to the regular print region is referred to as a“front-end print region”. A region which is located on the rear-end sideof the paper (the upstream side in the carrying direction) with respectto the regular print region is referred to as a “rear-end print region”.In the front-end print region, thirty raster lines are formed. Also, inthe rear-end print region, thirty raster lines are formed. In theregular print region, thousands of raster lines are formed, depending onthe size of the paper.

In the regular print region, there is regularity, for each set of rasterlines of a number corresponding to the carry amount (seven in thisexample), in how the raster lines are arranged. The raster lines fromthe first one through the seventh one located in the regular printregion shown in FIG. 8 are formed respectively by nozzle #3, nozzle #5,nozzle #7, nozzle #2, nozzle #4, nozzle #6, and nozzle #8, and the sevenraster lines following the seventh raster line are formed respectivelyby the nozzles in the same order as mentioned above. On the other hand,in the front-end print region and rear-end print region, there is nosimple regularity in how the raster lines are arranged in comparisonwith the raster lines in the regular print region.

Outline of Correction for Unevenness in Density

Regarding Unevenness in Density (Banding)

In this section, for convenience of explanation, a cause of unevennessin density that occurs in an image printed with monochrome printing isdescribed. In case of multi-color printing, the cause of unevenness indensity described below occurs for each color.

In the explanation below, a “unit region” means a virtual rectangularregion determined on a medium such as paper, the size and shape of whichare determined depending on print resolution. For example, in case thatthe print resolution is specified as 720 dpi (in the movementdirection)×720 dpi (in the carrying direction), a unit region is asquare region approximately 35.28 μm long and 35.28 μm wide 1/720 inch·1/720 inch). In case that the print resolution is specified as 360dpi×720 dpi, a unit region is a rectangular region approximately 70.56μm long and 35.28 μm wide 1/360 inch· 1/720 inch). If an ink droplet isideally ejected, the ink droplet lands in the center of this unitregion, then the ink droplet spreads on the medium, and a dot is formedin the unit region. One unit region corresponds to one of pixels whichimage data consists of. Since each unit region corresponds to eachpixel, pixel data of each pixel also corresponds to each unit region.

Furthermore, in the explanation below, a “row region” means a regionconsisting of a plurality of unit regions lined up in the movementdirection. For example, in case that the print resolution is specifiedas 720 dpi×720 dpi, a row region is a belt-like region having a width of35.28 μm (≈ 1/720 inch) in the carrying direction. If ink droplets areideally ejected intermittently from a nozzle moving in the movementdirection, a raster line is formed in this row region. One row regioncorresponds to a plurality of pixels lined up in the movement direction.

FIG. 9A is an explanatory diagram showing a state in which dots areformed ideally. In the figure, since dots are formed ideally, each dotis formed precisely in the unit region and each raster line is formedprecisely in the row region. Each row region is illustrated in thefigure as a region sandwiched by dotted lines, and in this case, is aregion 1/720 inch wide. In each row region, a piece of image which has adensity equivalent to coloring of the region is formed. Here, forconvenience of explanation, an image which has a constant density inorder to fix the dot-generation rate at 50% is printed.

FIG. 9B is an explanatory diagram showing how the variation in precisionof manufacturing among nozzles affects dot formation. Here, the rasterline formed in the second row region is formed closer to the side of thethird row region (the upstream side in the carrying direction) becauseof variations in the flying direction of ink droplets ejected fromnozzles. Also, since ink of ink droplets ejected to the fifth row regionis less in amount, dots formed in the fifth row region are smaller insize. Despite that, by definition, pieces of image having the samedensity should be formed in each row region, a variation in densityoccurs among pieces of image depending on row regions in which they areformed because of the variation in precision of manufacturing. Forexample, the piece of image in the second row region is formedrelatively light in color, and the piece of image in the third rowregion relatively dark in color. The piece of image in the fifth rowregion is formed relatively light in color.

Accordingly, in case of observing macroscopically a print imageconsisting of such raster lines, a streaky unevenness in density in themovement direction of the carriage is visually noticeable. Thisunevenness in density makes print image quality deteriorate.

FIG. 9C is an explanatory diagram showing how dots are formed by theprinting method of the present embodiment. In the present embodiment,for row regions which tend to be visually perceived darker in color,tone values of pixel data (CMYK pixel data) of pixels corresponding tothe row regions are corrected in order to form pieces of image lighterin color. Also, for row regions which tend to be visually perceivedlighter in color, tone values of pixel data of pixels corresponding tothe row regions are corrected in order to form pieces of image darker incolor. For example, in the figure, tone values of pixel data of thepixels corresponding to each row region are corrected in order toincrease the generation rate of dots in the second row region, todecrease the generation rate of dots in the third row region, and toincrease the generation rate of dots in the fifth row region. Thereby,the dot-generation rate of the raster line corresponding to each rowregion is changed, the density of the piece of image in the row regionis corrected, and thus unevenness in density in the entire print imageis suppressed.

Furthermore, in FIG. 9B, the piece of image formed in the third rowregion is darker in color, not because of effects of the nozzle thatforms the raster line in the third row region, but because of effects ofthe nozzle that forms the raster line in the second row regioncontiguous thereto. Accordingly, if the nozzle that forms the rasterline in the third row region forms a raster line in another row region,a piece of image formed in the other row region is not always darker incolor. In short, there are cases in which there is difference in densityof color even among pieces of image formed by the same nozzle if piecesof image contiguous to each of the above-mentioned pieces are formedrespectively by different nozzles. In this case, correction valuescorresponding only to each nozzle cannot suppress unevenness in density.Thus, in the present embodiment, tone values of pixel data are correctedbased on the correction values set for each row region.

Therefore, in the present embodiment, on an inspection process at aprinter manufacturing plant a printer prints a correction pattern, thecorrection pattern is read with a scanner, and a correction valuecorresponding to each row region, which is based on density of each rowregion in the correction pattern, is stored in a memory of the printer.The correction values stored in the printer reflects characteristics ofunevenness in density of each individual printer.

Then, under instructions by a user who has purchased the printer, theprinter driver reads the correction values from the printer, tone valuesof pixel data are corrected based on the correction values, print datais generated based on the corrected tone values, and the printerperforms printing based on the print data.

Regarding Process at Printer Manufacturing Plant

FIG. 10 is a flowchart showing a process for obtaining correctionvalues, which is performed on an inspection process after a printer hasbeen manufactured.

First, an inspector connects a printer 1 to be inspected to a computer110 in a plant (S101). The computer 110 in the plant is also connectedto a scanner 150, and has computer programs installed thereon, such as aprinter driver for having the printer 1 print a test pattern, a scannerdriver for controlling the scanner 150, and a program for obtainingcorrection values which is for carrying out image processing, analysis,or otherwise, of image data of correction patterns read by the scanner.

Second, the printer driver installed on the computer 110 causes theprinter 1 to print the test pattern (S102).

FIG. 11 is an explanatory diagram showing the test pattern. FIG. 12 isan explanatory diagram showing the correction pattern. In the testpattern, four correction patterns different in color are formed. Eachcorrection pattern consists of five belt-like patterns different incolor density (CD), one top ruled line, one bottom ruled line, one leftruled line, and one right ruled line. Each of the belt-like patterns isgenerated respectively from image data having a specific tone value,which is respectively 76 (30% CD), 102 (40% CD), 128 (50% CD), 153 (60%CD) and 179 (70% CD) in the order shown from left to right and becomesdarker as the belt-like pattern is located toward the right. These fivetone values (color densities) are referred to as the “designated tonevalues (the designated color-densities)” and are represented with therespective symbols: Sa (=76), Sb (=102), Sc (=128), Sd (=153), and Se(=179). Each belt-like pattern is formed by front-end printing, regularprinting and rear-end printing, and consists of raster lines in afront-end print region, raster lines in a regular print region, andraster lines in a rear-end print region. On printing of the correctionpattern, raster lines the number of which is equivalent to eight cyclesare formed in the regular print region though thousands of raster linesare formed in the regular print region in usual printing. Here, forconvenience of explanation, the correction patterns are printed by theprinting described in FIG. 8, and each belt-like pattern consists of 116raster lines in total: thirty raster lines in the front-end printregion, fifty-six raster lines (seven raster lines in each cycle×eightcycles) in the regular print region, and thirty raster lines in therear-end print region. The top ruled line is formed with the first oneof raster lines which the belt-like pattern consists of (the raster lineon the most downstream side in the carrying direction). The bottom ruledline is formed with the last one of raster lines which the belt-likepattern consists of (the raster line on the most upstream side in thecarrying direction).

Next, the inspector sets the test pattern printed with the printer 1 onthe scanner 150 by placing the test pattern on a document platen glass152 of the scanner 150 and lowering a lid 151. Then, the scanner driverinstalled on the computer 110 causes the scanner 150 to read thecorrection patterns (S103). The section below describes how thecorrection pattern of cyan is read (the correction patterns of othercolors are read in the same way).

FIG. 13 is an explanatory diagram showing a reading range of thecorrection pattern of cyan. The range within dot dash lines surroundingthe correction pattern of cyan is a reading range when the correctionpattern of cyan is read. Parameters SX1, SY1, SW1 and SH1, which are forspecifying this reading range, are preset on the scanner driver by theprogram for obtaining correction values. In case that this reading rangeis read by the scanner 150, the entire correction pattern of cyan can beread even if the test pattern is placed slightly out of position on thescanner 150. By this process, an image in the reading range in thefigure is read by the computer 110 as rectangular image data withresolution of 2880×2880 dpi.

Next, the program for obtaining correction values installed on thecomputer 110 detects an inclination θ of the correction pattern in theimage data (S104), and rotates the image data depending on theinclination θ (S105).

FIG. 14A is an explanatory diagram showing the image data on detectionof the inclination. FIG. 14B is an explanatory diagram showing how thelocation of the top ruled line is detected. FIG. 14C is an explanatorydiagram showing the rotated image data. The program for obtainingcorrection values obtains from the read image data pixel data of KHpieces of pixels from the top which are located KX1th from the left andpixel data of KH pieces of pixels from the top which are located KX2thfrom the left. The parameters KX1, KX2, and KH are preset in order forpixels obtained as mentioned above to include the top ruled line and toexclude the right ruled line and the left ruled line. In order to detectthe location of the top ruled line, the program for obtaining correctionvalues obtains respective barycentric positions of the tone values ofthe KH pieces of pixel data obtained: KY1 and KY2 The program forobtaining correction values calculates by the following formula theinclination θ of the correction pattern based on the parameters KX1 andKX2 and the barycentric positions KY1 and KY2, and rotates the imagedata based on the inclination θ calculated:

θ=tan⁻¹{(KY2−KY1)/(KX2−KX1)}

Next, the program for obtaining correction values installed on thecomputer 110 crops the image data in order to eliminate unnecessarypixels (S106). FIG. 15A is an explanatory diagram showing the image dataon cropping. FIG. 15B is an explanatory diagram showing a crop line withrespect to the top ruled line. In the same way as processed in S104, theprogram for obtaining correction values obtains from the rotated imagedata pixel data of KH pieces of pixels from the top which are locatedKX1th from the left and pixel data of KH pieces of pixels from the topwhich are located KX2th from the left. In order to detect the locationof the top ruled line, the program for obtaining correction valuesobtains respective barycentric positions of the tone values of the KHpieces of pixel data obtained, KY1 and KY2, and calculates an averagevalue of the two barycentric positions. A border of pixels nearest tothe position half width of a row region above the barycentric positionis determined as a crop line. In the present embodiment, since theresolution of the image data is 2880 dpi and the width of the row regionis 1/720 inch, the half width of the row region is equivalent to twopixels. The program for obtaining correction values crops pixels abovethe determined crop line. FIG. 15C is an explanatory diagram showing acrop line with respect to the bottom ruled line. In substantially thesame way as the top ruled line, the program for obtaining correctionvalues obtains from the rotated image data pixel data of KH pieces ofpixels from the bottom which are located KX1th from the left and pixeldata of KH pieces of pixels from the bottom which are located KX2th fromthe left, and calculates the barycentric position of the bottom ruledline. A border of pixels nearest to the position half width of a rowregion below the barycentric position is determined as a crop line. Theprogram for obtaining correction values crops pixels below the cropline.

Next, the program for obtaining correction values installed on thecomputer 110 converts the resolution of the cropped image data in orderto make the number of pixels in Y-direction equal to 116 (same as thenumber of raster lines which the correction pattern consists of) (S107).FIG. 16 is an explanatory diagram showing how to convert resolution. Incase that the printer 1 forms ideally the correction pattern consistingof 116 raster lines with resolution of 720 dpi, if the scanner 150 readsthe correction pattern ideally with resolution of 2880 dpi (with fourtimes as high resolution as the correction pattern), the number ofpixels in Y-direction of the cropped image data should be 464 (=116×4).However, actually, by effects of displacement caused when the image datais printed or read, there are cases in which the number of pixels inY-direction of the image data is not 464. Here, the number of pixels inY-direction of the cropped image data is 470. The program for obtainingcorrection values installed on the computer 110 converts resolution ofthe image data (performs a shrinkage process), at the rate of 116/470(=“the number of raster lines which the correction pattern consistsof”/“the number of pixels in Y-direction of the cropped image data”).Here, resolution is converted using the bicubic interpolation method. Asa result thereof, the number of pixels in Y-direction of the image dataafter resolution conversion is 116. In other words, the image data ofthe correction pattern with resolution of 2880 dpi is converted into theimage data of the correction pattern with resolution of 720 dpi. Thisconversion makes the number of pixels lined up in Y-direction equal tothe number of row regions, and one row of pixels in X-directioncorresponds to one row region on a one-to-one basis. For example, therow of pixels in X-direction located in the top corresponds to the firstrow region, and the row of pixels located immediately below theabove-mentioned row corresponds to the second row region. Since thisresolution conversion aims to make the number of pixels in Y-directionequal to 116, resolution conversion in X-direction (shrinkage process)does not necessary have to be performed.

Next, the program for obtaining correction values installed on thecomputer 110 measures respective densities of the five belt-likepatterns in each row region (S108). The section below describesmeasurement of density of the leftmost belt-like pattern in the firstrow region formed with 76 (30% CD) in tone value (measurement of densityof the other row regions in that belt-like pattern, as well asmeasurement of density of the other belt-like patterns in the first orother row regions, are performed in the same way).

FIG. 17A is an explanatory diagram showing the image data when the leftruled line is detected. FIG. 17B is an explanatory diagram showing howthe location of the left ruled line is detected. FIG. 17C is anexplanatory diagram showing a density-measuring range of the belt-likepattern in the first row region formed with 30% CD. The program forobtaining correction values obtains pixel data of KX pieces of pixelsfrom the left which are located H2th from the top, from the image datawhose resolution has been converted. The parameter KX is preset in orderfor pixels obtained as mentioned above to include the left ruled line.In order to detect the location of the left ruled line, the program forobtaining correction values obtains a barycentric position of tonevalues of pixel data of the KX pieces of pixels obtained. It is knownfrom the shape of the correction pattern that a W3 wide belt-likepattern formed with 30% CD exists X2 to the right of this barycentricposition (the location of the left ruled line). The program forobtaining correction values extracts, taking the barycentric position asa reference, pixel data within a range surrounded by dotted lines, whichexcludes two W4 wide ranges which are located at respective horizontalends of and within the belt-like pattern, and an average value of tonevalues of the pixel data within the range surrounded by the dotted linesis used as a measured value of the first row region with 30% CD. In caseof measuring density of the belt-like pattern in the second row regionformed with 30% CD, pixel data in a range one-pixel below the rangesurrounded by the dotted lines in the figure is extracted. In this way,the program for obtaining correction values measures densities of thefive belt-like patterns in each row region.

FIG. 18 is a table of values of measured densities of the five belt-likepatterns of cyan. In this way, the program for obtaining correctionvalues installed on the computer 110 creates the table of measuredvalues by associating, with each row region, the measured values ofdensities of the five belt-like patterns. For other colors, tables ofmeasured values are also created. In the explanation below, for acertain row region, measured values in the belt-like patterns with thetone values Sa through Se are represented with respective symbols: Cathrough Ce.

FIG. 19 is a graph showing the measured values of the belt-like patternsof cyan formed with 30%, 40% and 50% CD respectively. In each ofbelt-like patterns, variation in density occurs among row regionsdespite that the belt-like patterns are formed uniformly with therespective designated tone values. This variation in density among rowregions causes unevenness in density of a print image.

In order to eliminate unevenness in density, it is desirable that themeasured values are uniform in each belt-like pattern. Accordingly, thissection discusses a process for making measured values in a belt-likepattern with tone value Sb (40% CD) uniform. Here, an average measuredvalue Cbt across all row regions of the belt-like pattern with tonevalue Sb is determined as a target value for 40% CD. In the row region iin which a measured value is lighter in density than this target valueCbt, it is considered only necessary to correct the tone value so thatit becomes darker in order for the measured value of density to becomecloser to the target value Cbt. On the other hand, in the row region jin which a measured value is darker in density than this target valueCbt, it is considered only necessary to correct the tone value so thatit becomes lighter in order for the measured value of density to becomecloser to the target value Cbt.

Therefore, the program for obtaining correction values installed on thecomputer 110 calculates correction values corresponding to row regions(S109). This section describes how a correction value for the designatedtone value Sb of a certain row region is calculated. As described below,a correction value for the designated tone value Sb (40% CD) of the rowregion i in FIG. 19 is calculated based on measured values of the tonevalue Sb and tone value Sc (50% CD). On the other hand, a correctionvalue for the designated tone value Sb (40% CD) of row region j iscalculated based on measured values of tone value Sb and tone value Sa(30% CD). FIG. 20A is an explanatory diagram showing a target designatedtone value Sbt of the row region i for the designated tone value Sb. Inthis row region, a measured value Cb of density of the belt-like patternformed with the designated tone value Sb is smaller in tone value thanthe target value Cbt (in this row region, lighter in color than anaverage density of the 40% CD belt-like pattern). In case that theprinter driver causes the printer to form in this row region a patternwith density of the target value Cbt, it is only necessary to designatethe tone value based on the target designated tone value Sbt calculatedby the following formula (linear interpolation based on the straightline BC):

Sbt=Sb+(Sc−Sb)×{(Cbt−Cb)/(Cc−Cb)}

FIG. 20B is an explanatory diagram showing a target designated tonevalue Sbt of the row region j for the designated tone value Sb. In thisrow region, a measured value Cb of density of the belt-like patternformed with the designated tone value Sb is larger in tone value thanthe target value Cbt (in this row region, darker in color than anaverage density of the 40% CD belt-like pattern). In case that theprinter driver causes the printer to form in this row region a patternwith density of the target value Cbt, it is only necessary to designatethe tone value based on the target designated tone value Sbt calculatedby the following formula (linear interpolation based on the straightline AB):

Sbt=Sb−(Sb−Sa)×{(Cbt−Cb)/(Ca−Cb)}

After calculating the target designated tone value Sbt in this way, theprogram for obtaining correction values calculates a correction value Hbof this row region for the designated tone value Sb by the followingformula:

Hb=(Sbt−Sb)/Sb

The program for obtaining correction values installed on the computer110 calculates, for each of the row regions, the correction value Hb forthe tone value Sb (40% CD). Also, based on the measured value Cc and themeasured value Cb or Cd of each of the row regions, the program forobtaining correction values calculates, for each of the row regions, acorrection value Hc for the tone value Sc (50% CD). Also, based on themeasured value Cd and the measured value Cc or Ce of the each of the rowregions, the program for obtaining correction values calculates, foreach of the row regions, a correction value Hd for the tone value Sd(60% CD). Also, for other colors, three correction values (Hb, Hc, andHd) are calculated for each of the row regions.

There are fifty-six raster lines in the regular print region and thereis regularity for every seven raster lines. This regularity is takeninto consideration on calculation of the correction values in theregular print region.

When the program for obtaining correction values calculates thecorrection values of the first row region in the regular print region(the thirty-first row region in the entire print region), theabove-mentioned measured value Ca uses the average of the measuredvalues of the following eight row regions in the pattern formed with 30%CD: the first, eighth, fifteenth, twenty-second, twenty-ninth,thirty-sixth, forty-third, and fiftieth ones in the regular printregion. Also, when the correction values of the first row region in theregular print region (the thirty-first row region in the entire printregion) are calculated, the above-mentioned measured value Cb through Ceuses the respective averages of the measured values of the followingeight row regions in the patterns formed with the respective densities:the first, eighth, fifteenth, twenty-second, twenty-ninth, thirty-sixth,forty-third, and fiftieth ones in the regular print region. Based on themeasured values Ca through Ce, the correction values (Hb, Hc, and Hd) ofthe first row region in the regular print region are calculated asmentioned above. In this way, a correction value of a row region in theregular print region is calculated based on an average of measuredvalues of eight row regions, which appear at an interval of every sevenregions, in the pattern formed with each density. As a result thereof,in the regular print region, correction values are calculated only forthe first though seventh seven row regions, but the correction valuesare not calculated for the eighth through fifty-sixth row regions. Inother words, the correction values for the first though seventh sevenrow regions in the regular print region also serve as the correctionvalues for the eighth through fifty-sixth row regions.

Next, the program for obtaining correction values installed on thecomputer 110 stores the correction values in the memory 63 of theprinter 1 (S110). FIG. 21 is an explanatory diagram showing a table ofcorrection values of cyan. There are three types of tables of correctionvalues: for the front-end print region, for the regular print region,and for the rear-end print region. In each of the tables of correctionvalues, three correction values (Hb, Hc, and Hd) collectivelycorresponds to each one of row regions. For example, three correctionvalues (Hb_n, Hc_n, and Hd_n) correspond to the nth row region. Thethree correction values (Hb_n, Hc_n, and Hd_n) correspond to therespective designated tone values: Sb (=102), Sc (=128) and Sd (=153).Tables of correction values for the other colors are created in the sameway.

After the correction values are stored in the memory 63 of the printer1, the process for obtaining correction values has been completed. Then,the printer 1 is disconnected from the computer 110, and is shipped fromthe plant after other inspections of the printer 1. A CD-ROM in whichthe printer driver is stored is packaged with the printer 1.

Regarding Processes under Instructions by User

FIG. 22 is a flowchart showing processes under instructions by a user. Auser who has purchased a printer 1 connects the printer 1 to a computer110 owned by the user (as a matter of course, a different computer fromthe computer of the printer manufacturing plant) (S201, S301). Thecomputer 110 of the user is not required to be connected to a scanner150. Next, the user sets a packaged CD-ROM on a record/play device 140,and installs a printer driver (S202). The printer driver installed onthe computer requests the printer 1 to transmit correction values to thecomputer 110 (S203). The printer 1 transmits, on request, to thecomputer 110 tables of correction values stored in its memory 63 (S302).The printer driver stores the correction values transmitted by theprinter 1 in the memory (S204). As a result thereof, the tables ofcorrection values are created in the computer. After completion of theseprocesses, the printer driver is on standby until the printer driverreceives a print command by the user (NO in S205).

When the printer driver receives a print command by the user (YES inS205), the printer driver generates print data based on the correctionvalues (S206) , and transmits the print data to the printer 1. Theprinter 1 prints according to the print data (S303).

FIG. 23 is a flowchart showing processes in print data generation. Theseprocesses are performed by the printer driver.

First, the printer driver converts resolution (S211). The resolutionconversion is a process for converting image data (text data, picturedata, and the like) outputted by an application program into resolutionwith which the image is to be printed on paper. For example, if theresolution for printing the image on the paper is specified as 720×720dpi, the image data received from the application program is convertedinto image data with resolution of 720×720 dpi. The image data after theresolution conversion is data with 256 tone levels represented by RGBcolor space (RGB data).

Next, the printer driver converts colors (S212). The color conversion isa process for converting RGB data into CMYK data, which is representedby CMYK color space. This color conversion is performed by the printerdriver's referring to a table in which tone values of RGB data areassociated with tone values of CMYK data (Color Conversion Lookup Table:LUT). In this color conversion, RGB data of each pixel is converted intoCMYK data which corresponds to a color of ink. Data after the colorconversion is CMYK data with 256 tone levels represented by CMYK colorspace.

Next, the printer driver performs density correction (S213). The densitycorrection is a process for correcting a tone value of each pixel databased on the correction values corresponding to the row region which thepixel data belongs to.

FIG. 24 is an explanatory diagram showing how to correct a density ofthe nth row region of cyan. The figure shows how a tone value S in ofpixel data of pixels belonging to the nth row region of cyan iscorrected. A corrected tone value is S_out.

In case that a uncorrected tone value S_in of pixel data equals to thedesignated tone value Sb, the printer driver can form an image with thetarget value Cbt in the unit region corresponding to the pixel data ifthe printer driver corrects the tone value S_in so that it becomes equalto the target designated tone value Sbt. In short, if the uncorrectedtone value S_in of the pixel data equals to the designated tone valueSb, it is preferable that the tone value S_in (=Sb) is corrected toSb×(1+Hb) using the correction value Hb corresponding to the designatedtone value Sb. Also, if the tone value S of the pixel data before thecorrection equals to the designated tone value Sc, it is preferable thatthe tone value S_in (=Sc) is corrected to Sc×(1+Hc).

On the other hand, if the uncorrected tone value S_in is different fromthe designated tone value, the tone value S_out to be outputted iscalculated with linear interpolation as shown in the figure. In linearinterpolation in the figure, sections between the corrected tone valuesS_out (Sbt, Sct, and Sdt) corresponding to the designated tone values(Sb, Sc, and Sd) are interpolated with linear interpolation. However,the invention is not limited thereto. For example, a correction value Hcorresponding to a tone value S_in can be calculated by linearinterpolation between the correction values (Hb, Hc, and Hd)corresponding to the designated tone values, and a corrected tone valuecan be calculated with the formula S_in×(1+H) based on the correctionvalue H calculated.

Regarding pixel data of each of the first through thirtieth row regionsin the front-end print region, the printer driver performs densitycorrection based on the correction values corresponding to each of thefirst through thirtieth row regions, which are stored in the table ofcorrection values for the front-end print region. For example, regardingpixel data of the first row region in the front-end print region, theprinter driver performs density correction based on the correction value(Hb_(—)1, Hc_(—)1, or Hd_(—)1) corresponding to the first row regionstored in the table of correction values for the front-end printing.

Also, regarding pixel data of each of the first through seventh rowregions in the regular print region (each of the thirty-first throughthirty-seventh row regions in the entire print region), the printerdriver performs density correction based on the correction valuescorresponding to each of the first through seventh row regions, whichare stored in the table of correction values for the regular printregion. However, though there are thousands of row regions in theregular print region, the correction values corresponding to only sevenrow regions are stored in the table of correction values for the regularprint region. Accordingly, regarding pixel data of each of the eighththrough fourteenth row regions in the regular print region, the printerdriver performs density correction based on the correction valuescorresponding to each of the first through seventh row regions, whichare stored in the table of correction values for the regular printregion. Thus, regarding row regions in the regular print region, theprinter driver uses, repeatedly for every seven row regions, thecorrection values corresponding to each of the first through seventh rowregions. Since there is regularity for every seven row regions in theregular print region, the characteristic of unevenness in density isalso expected to appear in the same cycle. Therefore, using thecorrection values repeatedly in the same cycle reduces an amount of dataof the correction values to be stored.

Though the number of the row regions in the regular print region of thecorrection pattern is fifty six, the number of row regions in theregular print region of a print image to be printed by the user is muchmore than the above-mentioned number and is in the order of thousands.The rear-end print region consisting of thirty row regions is formed onthe upstream side of the regular print region in the carrying direction(the rear-end side of the paper).

In the rear-end print region, same as the front-end print region,regarding pixel data of each of the first through thirtieth row regionsin the rear-end print region, the printer driver performs densitycorrection based on the correction values corresponding to each of thefirst through thirtieth row regions, which are stored in the table ofcorrection values for the rear-end print region.

By the above-mentioned density correction, in a row region which tendsto be visually perceived darker in color, a tone value of pixel data(CMYK data) of pixels corresponding to that row region is corrected inorder to be lower. On the contrary, in a row region which tends to bevisually perceived lighter in color, a tone value of pixel data ofpixels corresponding to that row region is corrected in order to behigher. In addition, for other row regions in other colors, the printerdriver performs correction in the same way.

Next, the printer driver performs a halftoning process (S214).Halftoning is a process for converting data with a finer gradation oftone into data with a gradation of a tone that can be formed by theprinter. For example, by halftoning, data with 256 tone levels isconverted into 1-bit data with 2 tone levels or 2-bit data with 4 tonelevels. In halftoning, in order to enable the printer to form dots in ascattered manner, pixel data is generated using dithering, gammacorrection, error diffusion, and the like. When the printer driverperforms halftoning process, the printer driver refers to a dither tablein case of dithering, refers to a gamma table in case of gammacorrection, and refers to an error memory for storing diffused errors incase of error diffusion. The halftoned data has the resolutionequivalent to the above-mentioned RGB data (for example, 720×720 dpi).

In the present embodiment, the printer driver performs the halftoningprocess to pixel data with tone values corrected by density correction.As a result thereof, in a row region which tends to be visuallyperceived darker in color, the dot-generation rate of dots which araster line in that row region consists of decreases because tone valuesof pixel data of that row region are corrected in order to be lower. Onthe contrary, in a row region which tends to be visually perceivedlighter in color, the dot-generation rate increases.

Next, the printer driver rasterizes data (S215). Rasterizing is aprocess for rearrange the order of image data which is in a matrix form,into the order of transmission to the printer. Rasterized data isoutputted to the printer as pixel data contained in the print data.

When the printer prints based on the print data generated as mentionedabove, the dot-generation rate of a raster line in each of row regionsare changed and densities of pieces of image in the row regions iscorrected, and thereby unevenness in density in the entire print imageis suppressed as shown in FIG. 9C.

Though, in the explanation above, the number of nozzles and the numberof row regions (the number of raster lines) are reduced for convenienceof explanation, the actual number of nozzles is 180, and, for example,the number of row regions in the front-end print region is 360. However,processes performed by the program for obtaining correction values, theprinter driver, and the like are almost the same.

Effects on Gradient of Measured Values of Density

Regarding Gradient of Measured Values of Density

FIG. 25A is a graph of measured values of density of each row region inthe 30% CD belt-like pattern in case that a scanner is in normaloperation. When a scanner is in normal operation, the measured valuesare concentrated closely around an average measured value Cbt throughthe entire row regions. FIG. 25B is a graph of measured values ofdensity of each row region in the 30% CD belt-like pattern in case thata scanner is in abnormal operation. For example, if a guiding member 154of a scanner 150 (see FIG. 5A) is mounted obliquely, or if a document 5does not adhere to a platen glass since a lid 151 is not loweredsufficiently, optical distance between the document 5 and a line sensor158 changes depending on the location of a reading carriage 153 in thesub-scanning direction. If, because of this effect, outputs of the linesensor 158 change depending on the location of the reading carriage 153in the sub-scanning direction, there are cases in which measured valueschange depending on the location of each of row regions and a gradientexists throughout the measured values.

The section below describes effects in cases that a graph of measuredvalues slopes downward from left to right.

Regarding Effects on Gradient of Measured Values of Density (1)

FIG. 31A is a graph of correction values of a reference example. In thegraph of a reference example, unlike the above-mentioned explanation, acorrection value of each of row regions is calculated depending on ameasured value of each of the row regions, and an average measured valueof row regions each of which is in every seven regions is not used inorder to calculate correction values in the regular print region.

When the gradient of the measured values exists depending on thelocation of each of row regions, there also is a gradient in thecorrection values calculated based on the measured values depending onthe location of each of row regions. For example, regarding a row regioncloser to or in the front end, correction values are set in order todecrease a tone value S_in excessively (minus correction values) becausedensity is measured darker than the actual density. On the other hand,regarding a row region closer to or in the rear end, correction valuesare set in order to increase a tone value S_in excessively (pluscorrection values) because density is measured lighter than the actualdensity.

In this way, as a result that the gradient of the correction valuesexists depending on the location of each of row regions, the print imageof which the density has been corrected is printed gradually darker fromthe front end to the rear end. (However, deterioration of image qualityis not conspicuous because the difference in density between row regionscontiguous to each other is not serious).

Regarding Effects on Gradient of Measured Values of Density (2)

As mentioned above, in the regular print region, an average measuredvalue of eight row regions each of which is in every seven regions (forexample, eight row regions in the regular print region: the first,eighth, fifteenth, twenty-second, twenty-ninth, thirty-sixth,forty-third, and fiftieth ones) is used as a measured value whencorrection values are calculated.

FIGS. 26A and 26B show measured values arranged in order, which are usedon calculation of correction values. FIG. 26A is a graph in case that ascanner is in normal operation, and FIG. 26B is a graph in case that thescanner is in abnormal operation. Though the measured values in thefront-end print region or in the rear-end print region are the same asthe measured values shown in FIG. 25A or FIG. 25B, each of sevenmeasured values in the regular print region is an average measured valueof the eight row regions each of which is in every seven regions.

Here, in order to focus on a boundary between the front-end print regionand the regular print region, this description focuses on a measuredvalue of density of the thirtieth row region in the front-end printregion and a measured value (an average value) of density of the firstrow region in the regular print region (the thirty-first row region inthe entire print region).

On calculation of the correction value in the front-end print region,the measured value of density in the front-end print region is usedwithout calculation. Thus, when the scanner is in abnormal operation,the measured value which is measured darker than the actual density isused as it is on calculation of the correction value.

On the other hand, the average value of eight row regions each of whichis in every seven regions is used on calculation of the correction valuein the regular print region. Density of the first row region in theregular print region is measured darker than the actual density, and arow region is measured lighter in color as the region is located moreupstream in the carrying direction (for example, the fiftieth rowregion). Therefore, the average measured value of eight row regionswhich are the first, eighth, fifteenth, twenty-second, twenty-ninth,thirty-sixth, forty-third, and fiftieth ones in the regular print regionbecomes lower than the measured value of the first row region in theregular print region.

As a result thereof, despite that the measured values of density of thefirst through thirtieth row regions in the front-end print regioncontinuously slope, discontinuity occurs between the measured value ofdensity of the thirtieth row region in the front-end print region andthe measured value of density of the first row region in the regularprint region (the average value).

FIG. 31B is a graph of correction values when the gradient of measuredvalues exists. Here, an average measured value of row regions each ofwhich is in every seven region is used on calculation of the correctionvalues in the regular print region.

If the measured values are discontinuous at the boundary between printregions in this way, the correction values calculated based on themeasured values also become discontinuous. As a result thereof, itbecomes conspicuous that the image in the front-end print region on themost upstream side in the carrying direction (piece of image of thethirtieth row region in the front-end print region) is darker in colorin comparison with the image in the regular print region.

Also, discontinuity occurs between the measured value of density of theseventh row region in the regular print region (the average value) andthe measured value of density of the first row region in the rear-endprint region (without calculation). Also, the correction valuescalculated based on the measured values become discontinuous. As aresult thereof, it becomes conspicuous that the image in the rear-endprint region on the most downstream side in the carrying direction(piece of image of the first row region in the rear-end print region) islighter in color in comparison with image in the regular print region.

FIG. 27 is an explanatory diagram showing density around the boundarybetween the front-end print region and the regular print region anddensity around the boundary between the regular print region and therear-end print region. For convenience of explanation, image data whichis the source of this print image is image with uniform density. (Thoughdensity in each of the print regions is described to be constant forconvenience of explanation in FIG. 27, gradual change in density occurseven in each of the print regions in contemplation of effects describedin the above-mentioned “Regarding Effects on Gradient of Measured Valuesof Density (1)”).

In this way, in case that the gradient exists throughout the measuredvalues because of abnormal operation of the scanner, density correctionmakes the difference in density more conspicuous at the boundary betweeneach of print regions.

Regarding Effects on Gradient of Measured Values of Density (3)

FIG. 28 is a graph of measured values (average values) in the regularprint region which correspond to one cycle. The graph with a thin lineshows values in cases that the gradient of the measured values does notexist, and the graph with a thick line shows values in cases that thegradient of the measured values exists. In this graph, for convenienceof explanation, the gradient of measured values is shown larger thanthat of the above-mentioned graph.

As described above, in the regular print region, an average measuredvalue of eight row regions each of which is in every seven regions isused as a measured value when correction values are calculated. Here, incomparison between the average measured value of eight row regions whichare the first, eighth, fifteenth, twenty-second, twenty-ninth,thirty-sixth, forty-third, and fiftieth ones in the regular print regionand the average value of eight row regions which are the seventh,fourteenth, twenty-first, twenty-eighth, thirty-fifth, forty-second,forty-ninth, and fifty-sixth ones in the regular print region, theformer average value tends to be measured darker than the latter one. Inshort, the measured values (the average values) of density in theregular print region which correspond to one cycle slope downward fromleft to right.

If the measured values (the average values) change downward or upwarddepending on the location of each of row regions corresponding to onecycle, the correction values calculated based on the measured valueschange in the same way depending on the location of each of the rowregions. As a result thereof, in case of printing an image of which thedensity has been corrected, the image is printed gradually darker withinthe row regions corresponding to one cycle.

In the regular print region, the correction values of the row regionscorresponding to one cycle are used repeatedly for every seven rowregions. Therefore, when the correction value of the seventh row regionis used as a correction value of a certain row region, the correctionvalue of the first row region is used as the correction value of the rowregion contiguous to the above-mentioned region on the upstream side inthe carrying direction. As a result thereof, a relatively darker imageamong regions in one cycle (apiece of image of a row region to which thecorrection value of the seventh row region is applied) is contiguous toa relatively lighter image (a piece of image of a row region to whichthe correction value of the first row region is applied), and thedifference in density becomes more conspicuous. In addition, a partwhere this difference in density is conspicuous occurs repeatedly everyone cycle.

FIG. 29 is an explanatory diagram showing density of the regular printregion after density correction in case that a scanner is in abnormaloperation. For convenience of explanation, image data which is thesource of this print image is an image with uniform density.

Here, since the number of nozzle is reduced in this explanation forconvenience of explanation, it is possible that the difference indensity which occurs every one cycle is not conspicuous because thewidth of seven row regions, which corresponds to one cycle, is narrow,7/720 inch, and the difference in density is small between the first rowregion and the seventh row region within one cycle. However, inpractice, the actual number of nozzle is 180, the width of the rowregions which corresponds to one cycle is 179/720 inch, and thedifference in density is large between the first row region and the179th row region within one cycle. Therefore, the difference in densitywhich occurs every one cycle tends to become conspicuous.

In short, if the gradient exists throughout the measured values, streakson the print image become conspicuous despite density correction.

In the first embodiment described below, in order to prevent adverseeffects caused by the gradient of the above-mentioned graph of measuredvalues, the gradient of the graph of the measured values is modified. Onthe other hand, in the second embodiment, the correction values aremodified. Furthermore, in the third embodiment, in addition tomodification of the gradient of the graph of the measured values, thecorrection values calculated based on the modified measured values arefurther modified.

The First Embodiment (Modification of Measured Values)

In the present embodiment, in order to prevent adverse effects caused bythe gradient of a graph of measured values, the gradient of the graph ofthe measured values is modified and, correction values are calculatedbased on the modified measured values.

FIG. 30A is a graph of measured values before modification. The measuredvalues mentioned in this section are the same as shown in the graph inFIG. 25B.

A program for obtaining correction values obtains measured values ofdensity of each of row regions in the range of the twenty-first through106th row regions which is the range to be covered by the calculation.The reason why the first through twentieth row regions, which arelocated more downstream in the carrying direction than this range to becovered by the calculation, are excluded from the range to be covered bythe calculation is because it is possible that the first throughtwentieth row regions are measured lighter in density due to the factthat the first through twentieth row regions are located near the marginon the downstream side in the carrying direction of the correctionpatterns and therefore the image in the first through twentieth rowregions is read under the influence of the margin. Also, the reason whythe 107th through 126th row regions are excluded from the range to becovered by the calculation is because it is possible that the 107ththrough 126th row regions are measured lighter in density due to thefact that the 107th through 126th row regions are located near themargin on the upstream side in the carrying direction of the correctionpatterns and therefore the image in the 107th through 126th row regionsis read under the influence of the margin. On the other hand, the rangeto be covered by the calculation includes at least a part of thefront-end print region and the rear-end print region. This is forobtaining the gradient of measured values in contemplation of theseprint regions.

The program for obtaining correction values calculates a linear fittingline (a line for approximation) by the least-square method based on themeasured values of density of each of row regions which are locatedwithin the range to be covered by the calculation. In FIG. 30A, thelinear fitting line is shown with a thick line. Furthermore, the programfor obtaining correction values calculates the average value Cbt′ basedon the measured values of density of each of row regions which arelocated within the range to be covered by the calculation. This averagevalue Cbt′ is the above-mentioned target value Cbt.

Next, the program for obtaining correction values calculates, for eachrow region, a difference between a value of the linear fitting line ineach row region and the average value Cbt′, and the difference is usedas the modification value of the row region. Regarding each row regionoutside the range to be covered by the calculation, the linear fittingline is extended, a difference between a value of the extended line inthe row region and the average value Cbt′ is calculated, and thedifference is used as the modification value of the row region. Theprogram for obtaining correction values modifies the measured value ofeach of the row regions by subtracting the modification value from themeasured value of each of the row regions.

FIG. 30B is a graph of the measured values after modification. Thegradient of the graph is eliminated throughout the modified measuredvalues.

The program for obtaining correction values calculates the correctionvalues based on the modified measured values (S109), and stores thecalculated correction values in a memory 63 of a printer 1 (S110). Underinstructions by a user, a printer driver performs density correctionbased on the correction values calculated based on the modified measuredvalues and generates print data, and the printer prints based on theprint data.

In the present embodiment, since the modified measured values are aroundthe average value Cbt′ regardless of the location of each of rowregions, density is uniform throughout the print image even when, forexample, an image with uniform density is corrected by densitycorrection and is printed. In short, the present embodiment can suppressthe phenomenon that density of a print image after density correctionchanges gradually depending on the location of each of the row regionsthroughout the print image.

In the present embodiment, regarding the boundary between the front-endprint region and the regular print region, the measured values ofdensity in the front-end print region around the boundary and themeasured values (average values) of density of row regions in theregular print region around the boundary are around the average valueCbt′. As a result thereof, the measured values of row regions arecontinuous around the boundary between the front-end print region andthe regular print region. Also, the measured values of row regions arecontinuous around the boundary between the regular print region and therear-end print region. As a result thereof, in the present embodiment,even though density of row regions in the regular print region iscorrected based on correction values corresponding to one cycle, thedifference in density, which stands out in FIG. 27, is not conspicuousaround the boundary between each of print regions.

In addition, in the present embodiment, all of the measured values(average values) in the regular print region corresponding to one cycleare around the average value Cbt′. As a result thereof, both of themeasured value of density of the first row region and the measured valueof density of the seventh row region within one cycle are around theaverage value Cbt′ and are continuous. As a result thereof, in thepresent embodiment, even if the correction values corresponding to onecycle are used repeatedly, the difference in density which occurs everyone cycle as shown in FIG. 29 is not conspicuous.

The Second Embodiment (Modification of Correction Value)

FIG. 32A is an explanatory diagram showing correction values beforemodification. FIG. 32B is an explanatory diagram showing correctionvalues after modification. It should be noted that on calculation ofcorrection values in the second embodiment, measured values are notmodified as described in the first embodiment. Here, first, a front-endmodification value is described.

First, a program for obtaining correction values obtains correctionvalues of ten row regions, five each before and after a boundary betweeneach of print regions, in order to calculate the front-end modificationvalue. Here, the program for obtaining correction values obtains thecorrection values of the twenty-fifth through thirtieth row regions inthe front-end print region and the correction values of the firstthrough fifth row regions in the regular print region.

Then, the program for obtaining correction values calculatesrespectively an average value of the correction values of the five rowregions obtained from each of the print regions. Here, the program forobtaining correction values calculates respectively an average value ofthe correction values of the twenty-fifth through thirtieth row regionsin the front-end print region and an average value of the correctionvalues of the first through fifth row regions in the regular printregion.

Next, the program for obtaining correction values calculates adifference between the average value in the front-end print region andthe average value in the regular print region, and this difference isused as the front-end modification value. Here, the program forobtaining correction values calculates the front-end modification valueby subtracting the average value in the front-end print region from theaverage value in the regular print region.

Next, the program for obtaining correction values modifies thecorrection values by adding respectively the front-end modificationvalue to each of the correction values in the front-end print region. Asa result thereof, in FIG. 32B, each of the unmodified correction valuesin the front-end print region which are indicated with a dotted linebecomes each of modified values which are indicated with a solid line.In short, the correction values in the front-end print region aremodified in order to reduce the difference between the correction valuesin the front-end print region and the correction values in the regularprint region. As a result thereof, after modification of the correctionvalues, discontinuity between the correction values in the front-endprint region and the correction values in the regular print region canbe reduced.

Also, the program for obtaining correction values calculates a rear-endmodification value for a boundary between the regular print region andthe rear-end print region, and modifies the correction values by addingrespectively the rear-end modification value to each of the correctionvalues in the rear-end print region. As a result thereof, the differencein density between an image in the rear-end print region and an image inthe regular print region can be reduced.

Then, the program for obtaining correction values stores the correctionvalues modified as mentioned above in a memory 63 of a printer 1 (S110).Under instructions by a user, a printer driver performs densitycorrection based on the modified correction values and generates printdata, and the printer prints based on the print data.

In the present embodiment, for example, the difference in densitybetween an image on the most upstream side in the carrying direction inthe front-end print region (a piece of image of the thirtieth row regionin the front-end print region) and an image in the regular print regionbecomes small, and therefore the difference in density becomes lessconspicuous around the boundary between each of print regions.Furthermore, in the present embodiment, for example, the difference indensity between an image on the most downstream side in the carryingdirection in the rear-end print region (a piece of image of the firstrow region in the rear-end print region) and an image in the regularprint region becomes small, and therefore the difference in densitybecomes less conspicuous around the boundary between each of printregions.

The Third Embodiment (Modification of Measured Values and Modificationof Correction Values)

In the present embodiment, in order to prevent adverse effects caused bythe gradient of a graph of measured values, the gradient of the graph ofthe measured values is modified and correction values are calculatedbased on the modified measured values. Modification of the measuredvalues is not described because it is the same as the first embodimentmentioned above.

FIG. 30B is a graph of the measured values after modification. Thegradient of the graph is eliminated throughout the modified measuredvalues. A program for obtaining correction values calculates thecorrection values based on these modified measured values.

FIG. 33A is an explanatory diagram showing correction values beforemodification. FIG. 33B is an explanatory diagram showing correctionvalues after modification. Even if the correction values are calculatedafter modification of the gradient of the measured values, using anaverage value as the measured value of a row region in the regular printregion may cause discontinuity of the correction values at the boundarybetween each of print regions. If density is corrected based on thesecorrection values, the difference in density may become conspicuousaround the boundary between each of print regions. Therefore, in thesame way as the above-mentioned method of modifying the correctionvalues, the program for obtaining correction values calculates afront-end modification value around the boundary between the front-endprint region and the regular print region, and modifies the correctionvalues by adding respectively the front-end modification value to eachof the correction values in the front-end print region. As a resultthereof, the correction values in the front-end print region aremodified such that the difference between the correction values in thefront-end print region and the correction values in the regular printregion is reduced. The program for obtaining correction values alsocalculates a rear-end modification value around the boundary between theregular print region and the rear-end print region, and modifies thecorrection values by adding respectively the rear-end modification valueto each of the correction values in the rear-end print region. As aresult thereof, the correction values in the rear-end print region aremodified such that the difference between the correction values in therear-end print region and the correction values in the regular printregion is reduced.

Then, the program for obtaining correction values stores these modifiedcorrection values in a memory 63 of a printer 1 (S110). Underinstructions by a user, the printer driver performs density correctionand generates print data based on the modified correction values, andthe printer prints based on this print data.

In the present embodiment, since the modified measured values are aroundthe average value Cbt′ regardless of the location of each of rowregions, density is uniform throughout the print image even when, forexample, an image with uniform density is corrected by densitycorrection and is printed. In short, the present embodiment can suppressthe phenomenon that density of a print image after density correctionchanges gradually depending on the location of each of row regionsthroughout the print image.

In the present embodiment, regarding the boundary between the front-endprint region and the regular print region, the measured values ofdensity in the front-end print region around the boundary and themeasured values (average values) of density of row regions in theregular print region around the boundary are around the average valueCbt′. As a result thereof, the measured values of row regions arecontinuous around the boundary between the front-end print region andthe regular print region. Also, the measured values of row regions arecontinuous around the boundary between the regular print region and therear-end print region. However, even if the correction values arecalculated after modification of the gradient of measured values, thereare cases in which the correction values become discontinuous at theboundary between each of print regions because an average value is usedas a measured value of a row region in the regular print region. On theother hand, in the present embodiment, the correction values are furthermodified in order to reduce the difference between correction valuesaround the boundary between each of print regions. As a result thereof,the difference in density between an image in the regular print regionand images in the front-end print region and rear-end print regionbecomes small, and the difference in density becomes less conspicuousaround the boundary between each of print regions. Furthermore, in thepresent embodiment, even though density of row regions in the regularprint region is corrected based on correction values corresponding toone cycle, the difference in density, as shown in FIG. 27, is notconspicuous around the boundary between each of print regions.

In addition, in the present embodiment, all of the measured values (theaverage values) corresponding to one cycle in the regular print regionare around the average value Cbt′. As a result thereof, both of themeasured value of density of the first row region and the measured valueof density of the seventh row region within one cycle are around theaverage value Cbt′ and are continuous. As a result thereof, in thepresent embodiment, even if correction values corresponding to one cycleare used repeatedly, the difference in density, as shown in FIG. 29,which occurs every one cycle is not conspicuous.

Other Embodiments

Though the printer 1 and printing system 100 as one embodiment aredescribed above, the above-mentioned embodiments are provided forfacilitating the understanding of the invention, and are not to beinterpreted as limiting the invention. As a matter of course, theinvention can be altered and improved without departing from the gistthereof and the invention includes equivalent thereof.

For example, the above-mentioned printer 1 is a separate unit from thescanner 150. However, a multifunction machine into which a printer andscanner are incorporated can be used.

In the above-mentioned embodiments, the test pattern is printed and thetables of correction values are created on the inspection process inmanufacturing of the printer 1, but the invention is not limitedthereto. For example, a user who has purchased the printer 1 can print atest pattern with the printer 1, read the test pattern with the scanner150, and create tables of correction values. In this case, the printerdriver can include the program for obtaining correction values.

Furthermore, in the above-mentioned embodiments, one raster line isformed by one nozzle, but the invention is not limited thereto. Forexample, one raster line can be formed by two nozzles.

Comprehensive Description

(1-1) In the above-mentioned process for obtaining correction values, atest pattern is printed first (FIG. 10, S102). In printing of the testpattern, the dot formation process (FIG. 6, S003) is performedrepeatedly, and correction patterns (an example of a pattern) are formedon a sheet of paper (an example of a medium). Each of these correctionpatterns consists of a plurality of raster lines (an example of a dotrow) respectively formed in a plurality of row regions lined up in thecarrying direction.

Next, the correction patterns are read by a scanner 150 (S103, FIG. 13),and, after rotating (S105), cropping (S106) or resolution conversion(S107) if necessary, density of each of the row regions is measured(S108). Here, if the scanner 150 is in abnormal operation, output of aline sensor 158 changes depending on the location in the sub-scanningdirection of a reading carriage 153, and, as a result thereof, measuredvalues change depending on the location of each of the row regions (seeFIG. 25B). In case that the correction values are calculated based onthese measured values, the correction values do not reflectcharacteristics of a printer, and print image quality does not improveeven if density correction (S213) is performed with using thesecorrection values (see FIG. 27 and FIG. 29).

Therefore, the above-mentioned program for obtaining correction valuescalculates the modification values corresponding to the row regions,based on a measurement result of the row regions which are in a range tobe covered by the calculation (an example of at least a part of ameasurement result of density of a plurality of row regions).Specifically speaking, the program for obtaining correction valuesobtains the linear fitting line and the average value Cbt′ based on themeasurement result of row regions which are located in the range to becovered by the calculation of the linear fitting line (see FIG. 30A) andthe program calculates a difference between the value of the linearfitting line in each of row regions and the average value Cbt′, and thedifference is used as a modification value of the row region. Theprogram for obtaining correction values modifies the measured value ofdensity of each of the row regions based on the modification value (FIG.30B).

As a result thereof, even if the scanner 150 is in abnormal operation,unevenness in density reflecting characteristics of the printer can bemeasured almost in the same way as the measured values in normaloperation of the scanner 150 (FIG. 25A).

(1-2) In the above-mentioned embodiment, among the measured values ofthe first through 116th row regions, those of the twenty-first through106th row regions are specified as the range to be covered by thecalculation, and the first through twentieth row regions, which arelocated on the end section of the correction pattern on the downstreamside in the carrying direction, are excluded from the range to becovered by the calculation. Also, the 107th through 116th row regions,which are located on the end section of the correction pattern on theupstream side in the carrying direction, are excluded from the range tobe covered by the calculation. This is because it is possible that thoserow regions are measured lighter in density than the actual density dueto the fact that the end section of the correction pattern is locatednear the margin and measuring density of the row regions on the endsection of the correction pattern is affected by the margin.

(1-3) The above-mentioned program for obtaining correction valuesobtains the linear fitting line and the average value Cbt′ based onmeasurement result of the row regions which are located in the range tobe covered by the calculation of the linear fitting line (see FIG. 30A)and calculates a difference between the value of the linear fitting linein each of row regions and the average value Cbt′, and the difference isused as a modification value of the row region. As a result thereof,even if the gradient exists throughout the measured values, the gradientcan be modified.

However, the invention is not limited to the above-mentioned method ofcalculating modification values. For example, quadratic curveapproximation is available instead of linear approximation.

(1-4) In the above-mentioned embodiment, the linear fitting line iscalculated based on the least-square method. This enables to grasp thetendency of the gradient of the measured values. However, the inventionis not limited to the above-mentioned method of calculating the linearfitting line. In the least-square method, a linear fitting line thatminimizes the sum of the square of differences between the measuredvalues and the linear fitting line is calculated; however, insteadthereof, a linear fitting line that minimizes the sum of the differencesbetween the measured values and the linear fitting line can becalculated, for example.

(1-5) In the above-mentioned embodiment, it is desirable that the rangeto be covered by the calculation includes the measured values of densityof the row regions in the front-end print region and the measured valuesof density of the row regions in the regular print region if thecorrection pattern includes the dot rows formed in the front-end printregion by front-end printing (an example of the first printing) (anexample of the first dot row) and the dot rows formed in the regularprint region by regular printing (an example of the second printing) (anexample of the second dot row). As a result thereof, the measured valuesof density of the row regions in the front-end print region arereflected on calculation of the linear fitting line.

(1-6) It is desirable to use the method for measuring density in whichall components mentioned above are included because all advantages areachieved. However, it is not necessary to include all components. Inshort, it is essential only that a constitution enables to measureunevenness in density reflecting the characteristic of the printer.

(1-7) The above-mentioned program for obtaining correction valuesmodifies the measured values of density of the row regions andcalculates the correction values corresponding to the row regions basedon the modified measured values. When the print image is formed on thepaper (an example of a medium) under instructions by a user, the printerdriver performs density correction based on the correction values (S213)and generates print data, and the printer 1 forms each of the rasterlines which the print image consists of, based on the correction valuecorresponding to the row region in which the raster line is to beformed. As a result thereof, even if the scanner 150 is in abnormaloperation, the print image can be formed without unevenness in density.

A correction value can technically be associated with to a nozzle, notwith a row region. However, there are cases in which there is differencein density of color even among pieces of image formed by the samenozzle.

For example, there are cases in which there is difference in density ofcolor even among dot rows formed by nozzle #3 if rows contiguous to eachof dot rows formed by nozzle #3 are formed respectively by differentnozzles such as nozzle #1 and nozzle #4. Therefore, even if a specificcorrection value corresponds to nozzle #3 and a tone value of pixel datais associated with a raster line formed by nozzle #3 is corrected basedon the correction value corresponding to nozzle #3, unevenness indensity cannot surely be suppressed. Accordingly, the correction valuesare set corresponding to the row regions.

(1-8) The above-mentioned program for obtaining correction valuescalculates respective correction values corresponding to seven rowregions in the regular print region (see FIG. 21). When the print imageis formed under instructions by the user, the printer driver correctsthe tone values of pixel data of thousands of the row regions in theregular print region using repeatedly the correction valuescorresponding to the seven row regions, and, based on the corrected tonevalues, the printer driver performs the halftoning process and generatesthe print data.

In case that the scanner 150 is in abnormal operation, the difference indensity which occurs every one cycle repeatedly becomes conspicuous asshown in FIG. 29 if the correction values calculated based on unmodifiedmeasured values are used repeatedly. On the other hand, theabove-mentioned embodiment can suppress the occurrence of these streaks.

(1-9) In the above-mentioned regular printing, a carrying process with acarry amount of 7·D (an example of a predetermined carry amount) isrepeated, and then the print image is formed on the paper (an example ofa medium). Before the regular printing, the program for obtainingcorrection values, for example, calculates the correction valuecorresponding to the first row region in the regular print region, basedon the average measured value of eight row regions which are the first,eighth, fifteenth, twenty-second, twenty-ninth, thirty-sixth,forty-third, and fiftieth ones in the regular print region. In this way,the correction value corresponding to the nth row region in the regularprint region is calculated based on the measured value of density of thenth row region in the regular print region and the measured values ofdensity of another row region which is located an integer multiple ofthe carry amount of 7·D from the row region.

In case that the correction values are calculated in this way, ifdensity correction is performed based on the correction valuescalculated based on the unmodified measured values, there are cases inwhich the correction values become discontinuous between the first rowregion in the regular print region and the thirtieth row region in thefront-end print region contiguous to the first row region (see FIG. 26B)for example and in which the difference in density becomes conspicuousaround the boundary as shown in FIG. 27. On the other hand, theabove-mentioned embodiment can suppress the occurrence of thisdifference in density.

(1-10) In the above-mentioned embodiment, the correction valuecorresponding to a certain row region in the regular print region isused not only in order to correct the tone value of pixel data of thatrow region but also in order to correct the tone values of pixel data ofother row regions which are located integer multiples of the carryamount of 7·D from the row region.

As a result thereof, the number of correction values to be stored can bereduced.

(1-11) Especially, in case of using regularity, the number of correctionvalues to be stored can be reduced dramatically though there arethousands of row regions in the regular print region.

(1-12) Though there are thousands of row regions in the regular printregion when the print image is formed under instructions by the user,there are row regions the number of which corresponds to only eightcycles (fifty-six row regions) when the correction patterns are printed.As a result thereof, since the length of the correction patterns in thecarrying direction can be made short, a plurality of correction patternslined up in the carrying direction can be formed as shown in FIG. 13,for example.

(1-13) It is desirable to use the printing method in which allcomponents mentioned above are included because all advantages areachieved. However, it is not necessary to include all components. Inshort, it is only essential that a constitution enables to correctunevenness in density of a printer even if reading of correctionpatterns reflects characteristics of the scanner 150.

(1-14) As a matter of course, the above-mentioned embodiment disclosesmethods of calculating correction value as well as measuring methods andprinting methods.

(1-15) As a matter of course, the above-mentioned embodiment disclosesmethods of manufacturing printers (an example of a printing apparatus)equipped with a memory storing the correction values. According to thismethod of manufacturing a printer, a printer which stores the correctionvalues depending on the characteristics of individual printers can bemanufactured despite of abnormal operation of the scanner 150.

(2-1) In the above-mentioned process for obtaining correction values, atest pattern is printed first (FIG. 10, S102). In printing of the testpattern, the dot formation process (FIG. 6, S003) is performedrepeatedly, and correction patterns (an example of a pattern) are formedon a sheet of paper (an example of a medium). Each of these correctionpatterns consists of a plurality of raster lines (an example of a dotrow) formed in a plurality of row regions lined up in the carryingdirection. Next, the correction patterns are read by a scanner 150(S103, FIG. 13), and, after rotating (S105), cropping (S106), orresolution conversion (S107) if necessary, density of each of the rowregions is measured (S108). The program for obtaining correction valuescalculates the correction values for correcting density of a row regionin the front-end print region (an example of the first region) (anexample of the first correction value), based on the measured values ofdensity of each of the row regions (S109). The program for obtainingcorrection values also calculates the correction values for correctingdensity of a certain row region in the regular print region (an exampleof the second region) (an example of the second correction value), basedon the average of the measured values of density of row regions which isin every seven regions including the measured value of density of thecertain row region (S109, See FIG. 21). However, since the correctionvalues in the regular print region are calculated using the averagemeasured value of density of a plurality of row regions, the correctionvalues become discontinuous at the boundary between each of printregions. If density correction is performed with using these correctionvalues, the difference in density becomes conspicuous around theboundary between each of print regions.

Therefore, in the above-mentioned second embodiment and thirdembodiment, the correction values in the front-end print region aremodified in order to reduce the difference between the correction valuesin the front-end print region and the correction values in the regularprint region. This can reduce discontinuity between the correctionvalues in the front-end print region and the correction values in theregular print region. As a result thereof, the difference in densitybecomes less conspicuous around the boundary between each of printregions.

In the above-mentioned embodiment, a front-end modification value isadded to the correction values in the front-end print region. However,the invention is not limited thereto. For example, even by subtractingthe front-end modification value from correction values in the regularprint region, discontinuity can be reduced between the correction valuesin the front-end print region and the correction values in the regularprint region. However, in contemplation of modification of thecorrection values in the rear-end print region, it is desirable tocorrect the correction values in the front-end print region so that theybecome closer to the correction values in the regular print region.

(2-2) In the above-mentioned embodiment, the correction values of thefirst through thirtieth row regions in the front-end print region (anexample of the first correction value) are modified in order to reduce adifference between the average value of the correction values of thetwenty-fifth through thirtieth row regions in the front-end print region(an example of a plurality of the first correction values) and theaverage value of the correction values of the first through fifth rowregions in the regular print region (an example of a plurality of thesecond correction values). As a result thereof, the difference indensity becomes less conspicuous around the boundary between each ofprint regions.

(2-3) In the above-mentioned embodiment, the front-end modificationvalue is the difference between the average value of the correctionvalues of the twenty-fifth through thirtieth row regions in thefront-end print region (an example of a plurality of the firstcorrection value) and the average value of the correction values of thefirst through fifth row regions in the regular print region (an exampleof a plurality of the second correction value), and the correctionvalues of the first through thirtieth row regions in the front-end printregion are modified based on the front-end modification value (see FIG.32B and FIG. 33B). As a result thereof, the difference in densitybecomes less conspicuous around the boundary between each of printregions.

(2-4) However, this invention is not limited to cases of using anaverage value of correction values of a plurality of row regions aroundthe boundary between each of print regions. For example, the correctionvalues can be modified in order to reduce a difference between thecorrection value of the thirtieth row region in the front-end printregion (an example of the first correction value which is contiguous toa row region in the second region) and the correction value of the firstrow region in the regular print region (an example of the secondcorrection value which is contiguous to a row region in the firstregion).

(2-5) In this case, it is desirable that the front-end modificationvalue is the difference between the correction value of the thirtiethrow region in the front-end print region (an example of the firstcorrection value of the row region contiguous to the second region) andthe correction value of the first row region in the regular print region(an example of the second correction value of the row region contiguousto the first region), and that the correction values of the firstthrough thirtieth row regions in the front-end print region are modifiedbased on the front-end modification value.

(2-6) For example, when the scanner is in abnormal operation orotherwise, there are cases in which there is a gradient in the measuredvalues of density of each of row regions depending on the row regions(FIG. 25B). The density correction based on the correction valuescalculated using these measured values may cause adverse effects onprint image quality (see FIG. 29).

Accordingly, in the above-mentioned embodiment, the gradient of themeasured values is modified, and correction values in the front-endprint region and correction values in the regular print region arecalculated based on the modified measured values. This can reduceadverse effects caused by the gradient of the measured values.

(2-7) In the above-mentioned embodiment, the program for obtainingcorrection values obtains the linear fitting line and the average valueCbt′ based on a measurement result of the row regions which are locatedin the range to be covered by the calculation of the linear fitting line(see FIG. 30A) and calculates a difference between the value of thelinear fitting line in each of row regions and the average value Cbt′,and the difference is used as the modification value of the measuredvalue of the row region. As a result thereof, even if the gradientexists throughout the measured values, the gradient can be modified.

However, the invention is not limited to the above-mentioned method ofcalculating modification values. For example, quadratic curveapproximation is available instead of linear approximation.

(2-8) In the above-mentioned embodiment, the linear fitting line iscalculated using the least-square method. This enables to grasp thetendency of the gradient of measured values. However, the invention isnot limited to the above-mentioned method of calculating the linearfitting line. In the least-square method, a linear fitting line thatminimizes the sum of the square of differences between the measuredvalues and the linear fitting line is calculated; however, insteadthereof, a linear fitting line that minimizes the sum of the differencesbetween the measured values and the linear fitting line can becalculated, for example.

(2-9) In the above-mentioned embodiment, the front-end print regionmeans a region consisting of raster lines formed by front-end printing(an example of the first dot row) , and the regular print region means aregion consisting of raster lines formed by regular printing (an exampleof the second dot row) (see FIG. 8). According to the above-mentionedembodiment, the difference in density becomes less conspicuous aroundthe boundary between the front-end print region and the regular printregion.

(2-10) It is desirable to use the method for obtaining correction valuesin which all components mentioned above are included because alladvantages are achieved. However, it is not necessary to include all thecomponents. In short, it is only essential that a constitution enablesto obtain correction values reflecting the characteristics of theprinter.

(2-11) In the above-mentioned embodiment, after modification of thecorrection values by the program for obtaining correction values, when aprint image is formed on the paper (an example of a medium) underinstructions by a user, a printer driver generates print data by densitycorrection based on the correction values (S213), and thereby a printer1 forms the raster lines which the print image consists of, based on thecorrection values corresponding to row regions in which the raster linesare to be formed. As a result thereof, the print image can be formedwithout unevenness in density and the difference in density at theboundary between each of the print regions can be reduced.

A correction value can technically be associated with a nozzle, not witha row region. However, there are cases in which there is difference indensity of color even among pieces of image formed by the same nozzle.For example, there are cases in which there is difference in density ofcolor even among dot rows formed by nozzle #3 if rows contiguous to eachof dot rows formed by nozzle #3 are formed respectively by differentnozzles such as nozzle #1 and nozzle #4. Therefore, even if a specificcorrection value is associated with nozzle #3 and a tone value of pixeldata corresponding to a raster line formed by nozzle #3 is correctedbased on the correction value corresponding to nozzle #3, unevenness indensity cannot surely be suppressed. Accordingly, in the presentembodiment, correction values are set corresponding to row regions.

(2-12) As a matter of course, the above-mentioned embodiment disclosesmethods of manufacturing printers (an example of a printing apparatus)equipped with a memory storing correction values. According to thismethod of manufacturing a printer, a printer which stores correctionvalues depending on the characteristics of individual printers can bemanufactured.

1. A method for obtaining a correction value, comprising: reading by ascanner a pattern that consists of a plurality of dot rows formedrespectively in a plurality of row regions lined up in a directionintersecting a movement direction of nozzles; measuring density of eachof the row regions of the read pattern; calculating a first correctionvalue for correcting the density of a row region that is located in afirst region of the pattern, based on a measured value of the density ofthat row region that is located in the first region; calculating asecond correction value for correcting the density of a row region thatis located in a second region contiguous to the first region, based on ameasured value of the density of that row region and a measured value ofthe density of another row region that is located in the second region;and modifying at least one of the first correction value and the secondcorrection value in order to reduce a difference between the firstcorrection value and the second correction value.
 2. The method forobtaining a correction value according to claim 1, wherein: at least oneof the first correction value and the second correction value ismodified in order to reduce a difference between an average value of aplurality of the first correction values and an average value of aplurality of the second correction values.
 3. The method for obtaining acorrection value according to claim 2, wherein: the difference betweenthe average value of the first correction values and the average valueof the second correction values is determined as a modification value;and at least one of the first correction value and the second correctionvalue is modified based on the modification value.
 4. The method forobtaining a correction value according to claim 1, wherein: in order toreduce a difference between the first correction value of a row regionthat is located in the first region and is contiguous to the secondregion and the second correction value of a row region that is locatedin the second region and is contiguous to the first region, at least oneof the first correction value and the second correction value ismodified.
 5. The method for obtaining a correction value according toclaim 4, wherein: the difference between the first correction value ofthe row region that is located in the first region and is contiguous tothe second region and the second correction value of the row region thatis located in the second region and is contiguous to the first region isdetermined as a modification value; and at least one of the firstcorrection value and the second correction value is modified based onthe modification value.
 6. The method for obtaining a correction valueaccording to claim 1, wherein: the measured values of the density ofeach of the row regions are modified depending on the row regions; andthe first correction value and the second correction value arecalculated based on the modified measured values.
 7. The method forobtaining a correction value according to claim 6, wherein: whenmodifying the measured values depending on the row regions, a linearfitting line and an average value are obtained from the measured values,and the measured values of the density of the row regions are modifieddepending on a difference between a value of the linear fitting line ineach of the row regions and the average value.
 8. The method forobtaining a correction value according to claim 7, wherein: the linearfitting line is calculated based on the least-square method.
 9. Themethod for obtaining a correction value according to claim 1, wherein:the first region is a region including a first dot row formed by firstprinting; and the second region is a region consisting of a second dotrow formed by second printing that is different from the first printing.10. The method for obtaining a correction value according to claim 1,wherein: at least one of the first correction value and the secondcorrection value is modified in order to reduce a difference between anaverage value of a plurality of the first correction values and anaverage value of a plurality of the second correction values, thedifference between the average value of the first correction values andthe average value of the second correction values is determined as afirst modification value, at least one of the first correction value andthe second correction value is modified based on the first modificationvalue, in order to reduce a difference between the first correctionvalue of a row region that is located in the first region and iscontiguous to the second region and the second correction value of a rowregion that is located in the second region and is contiguous to thefirst region, at least one of the first correction value and the secondcorrection value is modified, the difference between the firstcorrection value of the row region that is located in the first regionand is contiguous to the second region and the second correction valueof the row region that is located in the second region and is contiguousto the first region is determined as a second modification value, atleast one of the first correction value and the second correction valueis modified based on the second modification value, the measured valuesof the density of each of the row regions are modified depending on therow regions, the first correction value and the second correction valueare calculated based on the modified measured values, when modifying themeasured values depending on the row regions, a linear fitting line andan average value are obtained from the measured values, and the measuredvalues of the density of the row regions are modified depending on adifference between a value of the linear fitting line in each of the rowregions and the average value, the linear fitting line is calculatedbased on the least-square method, the first region is a region includinga first dot row formed by first printing, and the second region is aregion consisting of a second dot row formed by second printing that isdifferent from the first printing.
 11. A printing method, comprising:reading by a scanner a pattern that consists of a plurality of dot rowsformed respectively in a plurality of row regions lined up in adirection intersecting a movement direction of nozzles; measuringdensity of each of the row regions of the read pattern; calculating afirst correction value for correcting the density of a row region thatis located in a first region of the pattern, based on a measured valueof the density of that row region that is located in the first region;calculating a second correction value for correcting the density of arow region that is located in a second region contiguous to the firstregion, based on a measured value of the density of that row region anda measured value of the density of another row region that is located inthe second region; modifying at least one of the first correction valueand the second correction value in order to reduce a difference betweenthe first correction value and the second correction value; when forminga print image on a medium, forming a dot row that is located in thefirst region and that the print image consists of, based on the firstcorrection value corresponding to the row region in which that dot rowis to be formed; and forming a dot row that is located in the secondregion and that the print image consists of, based on the secondcorrection value corresponding to the row region in which that dot rowis to be formed.
 12. The printing method according to claim 11, wherein:at least one of the first correction value and the second correctionvalue is modified in order to reduce a difference between an averagevalue of a plurality of the first correction values and an average valueof a plurality of the second correction values.
 13. The printing methodaccording to claim 12, wherein: the difference between the average valueof the first correction values and the average value of the secondcorrection values is determined as a modification value; and at leastone of the first correction value and the second correction value ismodified based on the modification value.
 14. The printing methodaccording to claim 11, wherein: in order to reduce a difference betweenthe first correction value of a row region that is located in the firstregion and is contiguous to the second region and the second correctionvalue of a row region that is located in the second region and iscontiguous to the first region, at least one of the first correctionvalue and the second correction value is modified.
 15. The printingmethod according to according to claim 14, wherein: the differencebetween the first correction value of the row region that is located inthe first region and is contiguous to the second region and the secondcorrection value of the row region that is located in the second regionand is contiguous to the first region is determined as a modificationvalue; and at least one of the first correction value and the secondcorrection value is modified based on the modification value.
 16. Theprinting method according to claim 11, wherein: the measured values ofthe density of each of the row regions are modified depending on the rowregions; and the first correction value and the second correction valueare calculated based on the modified measured values.
 17. The printingmethod according to claim 16, wherein: when modifying the measuredvalues depending on the row regions, a linear fitting line and anaverage value are obtained from the measured values, and the measuredvalues of the density of the row regions are modified depending on adifference between a value of the linear fitting line in each of the rowregions and the average value.
 18. The printing method according toclaim 17, wherein: the linear fitting line is calculated based on theleast-square method.
 19. The printing method according to claim 11,wherein: the first region is a region including a first dot row formedby first printing; and the second region is a region consisting of asecond dot row formed by second printing that is different from thefirst printing.
 20. A printing apparatus comprising: a memory; and aplurality of nozzles, wherein: the printing apparatus forms a patternthat consists of a plurality of dot rows formed respectively in aplurality of row regions lined up in a direction interescting a movementdirection of the nozzles, a scanner reads the pattern; the memory storesat least one modified correction value obtained by: measuring density ofeach of the row regions of the read pattern; calculating a firstcorrection value for correcting the density of a row region that islocated in a first region of the pattern, based on a measured value ofthe density of that row region that is located in the first region;calculating a second correction value for correcting the density of arow region that is located in a second region contiguous to the firstregion, based on a measured value of the density of that row region anda measured value of the density of another row region that is located inthe second region; and modifying at least one of the first correctionvalue and the second correction value in order to reduce a differencebetween the first correction value and the second correction value,thereby forming the at least one modified correction value.